First impression after 24 hours of comparing both headphones from an average listener:
Build Quality
Bose are well built, but they feel more fragile than the XM5 (or the QC45 from memory). The ear cups feel shallow and not as sturdy as the QC45 (which used glass-filled nylon). The headphone is really soft, the the lining seems very thin, wondering how it's going to hold. It's quite a small pair of headphones, big head might have problems with the fit. The adjustable hinge are really sturdy and quite hard to move up and down.
Comfort
The QC Ultra are replay comfortable, the clamping force is stronger than the XM5. The XM5 feel lighter on the head. It's a different fit, but both are equally comfortable. I think that the QC Ultra might be more comfortable on longer session because of the wider and squishier headband.
Connectivity
Multipoint works very well from what I've tested so far. I had some problems with the QC 45 stuttering and skipping when connected to both my iPhone and my Macbook, but it's not the case with the QC Ultra.
Sound Quality
Excellent sound quality, on par with Sony and maybe better in the low end than the XM5. It's punchier and more I feel like it has more presence. The output volume is on par with the Sony, the QC45 are quieter in comparaison.
ANC
On par with the XM5 from what I have tested in my house. The Sony seems to have a better passive isolation, but the ANC on the Ultra does a great job of blocking the noise, so they are similar in that department. The QC Ultra has some white noise when nothing is playing, the XM5 are really silent it that regard.
I'm going to compare them further, but right now I feel that there is not enough differences in the headphones for the QC Ultra to replace my Sony WH100-XM5.
Videos
I had Sony WH-1000XM5 for 6 months. After that, unfortunately, right earcup stopped working. I was compensated the money and decided to buy a Bose QuietComfort Ultra as I thought it'd be an upgrade. I used Bose QC Ultra for 2 days which was enough for me to make a comparison between them.
Sony
Pros:
Good sound.
Good ANC, no white noise at all.
Great buttons: easy to alter sound level (it changes sound by the same small rate I change it using laptop keyboard keys which is great for me), switch between ANC and ambient mode (no need to press and hold, just press).
Fast charging: 30 minutes to full charge. If you need it quick, 3 minutes charging gives you several hours of headphones working.
Decent ambient mode, fast to enter. It feels a little muffled, but it's still great. There is absolutely no hiss.
Almost 0 (0.4 ms) latency on wired connection.
Can work wired without turning the headphones on (this saves battery). Though in that case ANC doesn't work and I personally don't see use cases for this. ANC is great during playing games - it blocks fans, neighbours and other noises which disrupt gaming experience.
Pleasant voice assistant: it's only a bit loud which is not a problem. Music when turning on/off and changing ANC on / ANC off / ambient mode is great and not loud at all.
Astonishing call quality. In noisy enironments, you're gonna be heard clearly and loud enough, blocking the noises around.
Cons:
Terrible headband: top of my head is aching bad after long sessions because the headband is too narrow. This is a well-known issue for a lot of people which keep them from buying these headphones. This was the only reason I wanted to change to Bose QC Ultra.
Terrible grip: the headphones are not sitting tight, every head turn makes them shift their position. When you move, the headphones are shifting a bit. Also, it makes ANC to recalibrate, as a result you hear surrounding noises better. But overall it's not that bad and won't give you much trouble.
Multi-connection is not working the best: when I'm playing games on a laptop and want to switch to listen to memes on the phone, the sound moves to phone for a couple of seconds and then gets back to the computer even though I still listen to the phone. Music is playing on both laptop and phone, so the headhones might not understand which device to prioritise and maybe just sticks to the laptop as a default behaviour implemented in them. So perhaps it's not a drawback. However, Bose sticks to the phone when I turn it on and then gets back to the laptop when I turn the phone off - this is the behaviour I'd expect to see.
A couple of times the headphones turned off during playing games (interesting thing that it only happened during playing Alan Wake (official, not a pirate version) and once playing Overwatch. It's weird. Yet it happened only like 5 times in general in 6 months).
It takes much space to carry them as they are big and don't fold.
Bose
Pros:
The headphones fold, so they take smaller amount of space.
ANC blocks external noises well.
Good switching between devices (multi connection).
Pretty comfortable, wider headband and bigger earcups.
Good sound quality (unless you hear white noise during listening, as it completely nullifies this pro).
Immersive audio.
Cons:
Constant hiss (background white noise) from ANC (and it doesn't help to further attenuate noises meaning you hear noises, which ANC cannot block completely, as if without the hiss. But the thing is that it makes noise even when you are in a quiet environment which is strange as you come to ANC for the purpose of being in quietness, so hissing ANC in a quiet room is questionable. It's ANC, it shouldn't give noise to cancel other noise).
When you listen to music in a normal sound level (not high) or a low one, background hiss is audible, especially when you're in a quiet room. It undermines the experience as you hear noise added to music and don't hear its details.
Ambient mode adds more hiss to already terrible white noise from ANC.
Not the best ANC (I still hear noises I wouldn't expect to hear). QC Ultra is said to have the best ANC. I don't have appropriate conditions to fully test it. But according to rtings, it only has 1 - 3 dB difference in noise attenuation (for example, Mid range (people talking): Sony: -28.25 dB, Bose: -29.81 dB). So ANC is pretty much the same as Sony's regarding noise attenuation, but Sony doesn't have hiss at all. You really want to hear constant hissing all the time even when being in a quiet environment for this extra 3 dB attenuation at max?
Buttons are not really convenient: you cannot adjust sound gradually (only by big chunks), you have to press and hold a button to turn ambient mode on which is not OK when someone starts to talk to you and with Sony you can engage in a conversation right away (because of a quick button press) whereas for Bose you have to hold the button and wait for the headphones to react. Also clicking a button has its own noise (clicking noise, not from the headphones) which some people may find irritating. Also, double-press and triple-press don't really seem convenient nowadays to skip a song or return to the previous one.
The headphones are too loud for my laptop. I have to make all sounds (in all applications) lower but even after that, the overall sound is at 6 max (out of 100!). The only solution people found on the internet is to turn down frequencies using ANC. I don't need to say this will result in a worse sound, right? And this is for the premium so-called the best headphones where you expect to get the best sound possible for that amount of money, yet you have to sacrifice it for being able to control sound level. For Sony, I had sound at 16. If I wanted music to be really quiet, I set it at 8.
When you walk, with every footstep you hear unpleasant thud due to ANC recalibrating and trying to block the footsteps. This one really drives me crazy. Sony recalibrates as well but without that sound of knocking at all.
Terrible sound of turning on and off (I really hate it!). Also it's too loud and no way to change it.
No fast charging - you need to spend several hours charging the headphones.
When you use mic in noisy environments, noise around you will be blocked, but so is your voice - it will be quiet and you won't be heard clearly.
It has 35 ms latency on wired connection. For a casual user, this is ok, I believe (though I don't really play online games and not sure whether it's appalling). But for a professional who works with sound editing, this is a deal breaker.
In order to work wired, the headphones must be turned on. Have a meeting but your headphones are dead? Poor you. This is generally not something that is very useful, yet in some situations you may encounter issues because of Murphy's law and the fact they don't have fast charging. BTW, there is no way to turn ANC off. So when you can plug Sony in while they're off and they'd work normally but without ANC (the battery would not drain in that case), Bose is not gonna work like this. You're gonna experience ANC with its hiss all the time even in complete quietness.
App problems: I was logged out of Bose app 2 times in 2 days. Maybe some unlucky coincidence, but it was super annoying and I've never had such an issue with Sony.
Price is bigger than for Sony. Why it costs more than Sony, given it comes behind in a lot of regards?
Sony and Bose:
Sony has smaller latency both wired and via Bluetooth (from specs, though not noticeable for me as a laymen).
Sony charge takes 30 min (because of quick charge support), Bose takes 3 hours to charge.
Sony changes sound level by a small amount (for my laptop, it's by 2 points out of 100). Bose - strange pretty big amounts (for my laptop it's by 7 points; I set Bose headphones at 6-th level out of 100 due to their loudness and changing sound for 7 points is huge and not practical).
Bose have deeper earcups. Though I didn't have any issue with Sony whatsoever.
Bose case for headphones is 1.5 times smaller than Sony. I should say that Sony headphones require quite much space.
Bose has immersive audio which I didn't find revolutionary but it's interesting anyway. Sony has 360 Reality Audio but I happened to never have used it. I suspect it's available from the app directly. If that so, Bose and Sony may be the same in this regard.
Summing up:
Bose QC Ultra: expensive headphones that have a lot of drawbacks which are not told on the Internet but obvious from the beginning when you get them. They are more portable due to smaller case, its ANC blocks external noises well, multi-connection works perfectly. They also seem to be comfortable (that's a known advantage of Bose in general as I understand) as they have wider headband than Sony does, but I didn't have opportunity to check because I was absolutely shocked by the constant hiss which prevented me from wanting to use them. I love quietness and I absolutely don't understand that everlasting noise from ANC when I'm in a peaceful environment, especially given that Sony eliminates noise by the same amount without giving its own noise. Furthermore, be ready to experience thuds when walking.
I hadn't understand why Sony was told to have the best ANC on the market. It didn't work perfectly and I still heard noises I didn't want to. When I got Bose QC Ultra, I finally got it. Sony has de-facto the best ANC that the mankind created so far and it didn't yield any noise at all. In quietness, you experience quietness, so do you with a moderate noise. In noise, it attenuates it as much as it can, producing no noise by itself as well. So it's absolutely true that Sony XM5 has the best ANC for now. I don't take Bose serious with their constant hiss.
Moveover, Ambient mode doesn't produce noise as well for Sony which is absolutely not true for Bose when the hissing is further amplified.
Sony, in my opinion, is the best ANC headphones existing at the moment. They have good sound, astonishing quiet ANC, great mic, convenient buttons, fast charging, cool ambient mode, almost 0 latency when wired (and can work wired without using its battery) and pleasant voice assistant when you press buttons.
It has only 2 drawbacks where Bose QC Ultra has an edge: comfort and case size.
Comfort. When I work from office, I wear the headphones all the day to block the noise. Though Sony XM5 cannot completely block constant people chatter (seems that QC Ultra won't be able as well). The issue is that the Sony presses on top of my head too much and it hurts quite a lot. When using them all the day everyday, this preasure is disgusting. Though it's worth mentioning that while a lot of people experience that, some people, on the contrary, say that XM5 is super comfortable. So your mileage may vary and you really should try using them in your conditions with your unique head to understand whether they're OK for you. But overall, the problem exists and top of your head is gonna hurt if you're not lucky with the head structure.
Case size. Travelling using the headphones means you need to get the case with you. Sony takes quite a lot of space due to the fact they don't fold. Meanwhile, Bose have a case which is 1.5 times smaller that will allow you to find the required space easier.
Overall, Sony, from my experience, is an almost perfect pair of headphones that have lots of advantages. It's ANC is on parity with Bose, with the latter being slightly better but noisy. It has only 2 cons: comfort and case size. Bose, on the other hand, for the higher price tag, delivers better portability, maybe cool comfort, good ANC. With these pros, there are a lot of disadvantages compared to XM5: mic quality, constant hiss, inconvenient buttons, thud noice when walking, too loud sound, terrible sound of voice assistant, lack of fast charging, underlying latency, no wired work without turning them on.
I absolutely cannot justify the price tag when comparing to features of the rivals such as Sony XM5.
Personally, I wanted to switch from XM5 to QC Ultra only for 2 reasons: comfort and ANC. I thought Bose will have a better ANC and I was terribly wrong. Noises that XM5 cannot block, QC Ultra cannot block either, yet it adds other noises like thud during walking and constant hiss. Bose seems to indeed be better regarding comfort. But all of other drawbacks cannot justify the switch for the comfort alone. Now I perfectly understand that comfort headphones which I don't want to wear is worth nothing to me. Between them two, Sony XM5 is my clear winner.
Links:
Comparing Sony XM5 and Bose QC Ultra
Bose QC Ultra White Noise / Static
Minimum volume is too loud solutions