S3 - Auto mode - regulates solution and suction based on how dirty the floor is, talks to you about what mode it is in and when the tank is full, charges by plugging cord directly into the back, has a self cleaning cycle. S5 - Same as S3, but bigger water tanks, more information on the display, better edge cleaning on one side, more advanced self-cleaning feature. There is an S5 Blue model that includes a base that the charging cord plugs in to (so the cord can stay put), S7 Pro - Same as S5 plus bigger tanks, better edge cleaning on both sides, bigger display, wheels are powered for easier use, base included. S7 Steam - Same as S7 Pro but does not use cleaning solution - cleans and sanitizes with steam. also a bigger tank, base included. These are all sold as a one step floor solution - vacuum and mop at once. And, you can do that. But the more debris you pick up, the bigger mess you have cleaning out the tank and filters. That is such a yucky mess, that I now vacuum first (which does a better job anyway), and then use the mop. For me, that is less work and time that dealing with all the pet hair and stuff clogged in the mop. In my opinion, the S5 comes out as a really good value - it ticks a lot of boxes. As you go up the line you do get nice capabilities like even bigger tanks and better edge cleaning - but the price jumps up dramatically. That being said, the S7 Steam is actually around $200 cheaper than the S7 Pro, so that model is giving you steam cleaning without pricing up in the stratosphere. Note - if you are in the US, there should be an Amazon Prime Day sale coming up in October. These are often discounted then. And other retailers (Walmart, Best Buy) often match those prices. And then of course Black Friday is a month after that. Answer from EddieRyanDC on reddit.com