This is such a wide subject it really isn't one you can answer with a simple one is better than the other answer.

Standing alone, the blow side of a fan does produce a more concentrated, faster moving, and more turbulent "river" of air compared to the intake side where air is drawn almost equally from all directions. You can test this easily enough with pretty much any fan. Put you hand in front of the blow side and you will feel the airflow and cooling effect. Put your hand behind and the effect is much harder to detect.

The turbulence also greatly improves the efficiency of the heat transfer. Turbulence is in fact your friend.

So from those points of view alone, the blow side does appear the better cooling side.

However, it is not just about the fan.

The geometry of the heat-sink chosen also greatly affects the performance of the fan. A rotary fan slapped on top of your typical linear finned heat-sink will actually be quite inefficient. In fact the region directly under the centre of the fan will get virtually no air movement at all. This of course is unfortunate, since that is normally where the thing you are trying to cool is located.

Further, unless the fins are quite deep the airflow is badly distributed in general. Too shallow, and the resultant back-pressure can actually "stall" the fan. In those circumstances, installing the fan in the "suck" direction can actually improve the situation since the air will enter the sides of the heat-sink more linearly to fill the void in air pressure created by the fan.

Arguably, the heat-sink shown above might be more efficient with longer fins and the fan mounted at one end.

Better designs use radial heat-sinks like the one below. As you can see, the style here is radially symmetric to the airflow on the entire circumference of the fan and consequently delivers a more even heat transfer around the central core.

However, even with this style, the core itself is still badly ventilated. As such it is usually manufactured as a solid high thermal conductance core which acts as a heat-pipe. Even then, looking at the image below, the area around the core in the square section that touches the chip actually is an air void that is quite inefficient. A better design would have that area filled with metal in a rounded conical structure. However, that would of course be impossible to extrude.

If fact materials and surface preparations also make a huge difference in heat-sink design. Highly thermally conductive materials are obviously best, but the surface should also be smooth enough not to allow pockets of air to form or to grab at dust particles, but also not so smooth that air passes too easily over it.

One could of course spend years getting that little formula perfect, but in general you don't want a high polish chrome heat-sink. Sandblasted aluminum, or gold coated sandblasted copper, if you can afford it, would work a lot better.

Another serious issue is contamination.

Dust and dirt is going to get into your fan and your heat-sink. Over time this builds up and severely degrades the performance of the unit. It is therefore prudent to design your fan and heat-sink arrangement to be as self flushing as you can.

This is where a blower fan usually wins out. With controlled airflow and if the air coming in can be kept clean, it tends to blow dust out of the heat-sink. Which brings me to the next point.

Air Sourcing and Removal

You can spend thousands of dollars developing the perfect arrangement of fan and heat sink and it will all be for naught if you do not deal with the rest of the air around your cooling system, especially in a tight enclosure.

The heat not only has to be removed from your device to air, but that hot air then needs to be removed from the vicinity. Failing do to so will just recirculate the hot air and thermal failure will still occur on the device you are trying to protect.

As such your cabinet needs to be vented and you should also include cabinet fans to draw in cool air from outside the enclosure. These fans should always include removable mesh and or foam filters to control the amount of ambient dust sucked into the unit. Open grill type exhaust panels are acceptable, however, for best operation a positive pressure should be maintained within the cabinet so airflow is maintained in the out direction to again limit contamination entry.

Special Cases

Wherever the unit is to be installed in an extreme environment special measures need to be taken. High dust environments like floor mills etc., or high ambient temperature environments will require either ducted air direct to the chassis, or a sealed unit and a two stage, possibly liquid, cooling system.

Critical Cases

If your system is controlling something critical then it is prudent to include thermal sensing and possibly active fan control as part of your heat-sink system. Such systems should include the feature of going into a safe state and warning the user to clean the filters or otherwise reduce the ambient heat around the system when necessary to prevent critical failures.

One More Point

You can spend a half years development money getting the best heat-sink design in the world with expensive fans and a perfect air distribution system all locked down then burn out devices for the lack of 2 cents worth of thermal compound.

Getting the heat from the device you are trying to protect into the heat-sink can often be the weakest point in the system. Components not properly mounted to the heat-sink with an appropriate thermal bonding material kills more units than the rest of the issues combined.

Your manufacturing process and procedures should be developed to give those aspects first priority.

For example, if say you are using three or four TO220 style transistors mounted to a single heat-sink, it is prudent to mechanically mount them to that heat-sink, and if appropriate, the heat-sink to the board, BEFORE going through the soldering process. This ensures the thermal connection takes priority.

Either thermally conductive pastes, creams, gels and or electrically isolated thermal pads should always be included between device and heat-sink to fill any air gaps caused by non-flatness, or bumps on either the device or the heat-sink surface.

And keep it clean. A contaminate the size or a grain of salt, or even a stray hair, can cause thermal failure.

A solution involving a fan will also include some kind of heatsink. It's not like you are removing a heatsink and replacing it with just a fan, the fan will be integrated with some kind of heatsink as well. Generally though, with good airflow provided by the fan heatsinks can often be a lot smaller.

The only benefit to a heatsink-only arrangement is less noise. It will not improve the reliability of the underlying hardware and in the case that the enclosure the heatsink is in has poor airflow could make things a lot worse.

Yes the fan is "yet another" component that can fail but what it gives you is the active movement of air which draws heat off of the heatsink, without that active movement you rely exclusively on convection to get cool air to the heatsink fins.

Take the graphics card for example...

If the graphics card heatsink is facing downwards (as is common in a desktop PC) then you will end up with hot air pooling underneath it, potentially raising the average temperature by a significant amount. Out of preference you want the heatsink fins to be standing upwards so that hot air can immediately rise off of it and cool air be pulled in. By necessity going to a heatsink only arrangement means you need to look very carefully at component placement and orientation, something you care less about with fans pushing the air around.

Heatsinks alone also cannot keep a device as cool as a heatsink and fan arrangement. Generally in order to remove a fan you need to have several times the surface area to radiate heat off of. With high power devices that can make heatsinking impractical. I've seen only a couple of high end fanless current generation cards and they all took up 3-4 slots just to get a large enough heatsink. They were either massive metal beasts, or were from the bottom of the range.

These considerations affect how and where you would use a fanless card. You would not use it in a bleeding edge gaming rig because it would take up most of the case, you could use it in a small home theatre PC so long as you had some airflow and didn't expect to play next-gen games on it, you could easily use it in a workstation PC quite happily.

Fans, generally, are highly reliable and dirt cheap to replace. The technology is mature and quality is generally pretty high across the board.

I'd only go fanless for a machine that I knew was pulling limited duty.

As to reliabilityfor fanless components you are basically exchanging the reliability of the fan for the belief (or hope) that the person who specified the heatsink has done their homework properly. That person has to have spent time working out the heat capacity of the metal, made sure that the thermal resistance of the block is low enough and that it has a large enough area to radiate that heat away under higher ambient temperatures. Granted a lot of that work has to be done for fanned parts, but fans can remove heat a lot better than stagnant air.

The actual component reliability should be the same as they are the same basic parts. The difference comes when the heatsink is underspecified or slightly too small.