Pick Air Cooling every single time that your CPU, GPU, and Case combination allow it. It's practically no maintenance, uses less power, has no risk of leaking, won't degrade over time like water cooling will (even sealed AIO's evaporate over the years), has less moving parts, longer warranties (usually), and it's cheaper (more or less). Modern air coolers are just as quiet as liquid coolers too.

So when DO you want water cooling?

Well, I thought I would never water cool a PC until recently. Take a look at one of my PC's here. I needed to use this case because it replaced a big desktop footprint with a "tall" case. It takes up less actual space on my desk than even the smallest of SFF cases that can still support this kind of prosumer level hardware. The motherboard is rotated 90 degrees, so if this was a typical ATX style PC case the "front" where the intake is would be the bottom of my case if that makes sense.

Thermaltake says the desired way to use this case is to have the right side be the intake, top exhaust, and the left side is an exhaust. Which makes no sense to me because the gpu is drawing IN from that left side (they even have a filter on that side's mesh, usually there are not fans there like in my image), but ok fair enough. It's really no different from a typical case tossed on it's side, except I had one big issue.

This founders edition 5090 blows THROUGH both sides of the card. If this was any of the third party 5090's the bottom would blow through, under that fan on the bottom, and the top of the 5090 would blow into the PCB of the card, so really nothing is getting pushed "through" the GPU to the CPU. But the Founder's Edition does, and so my big giant Scythe Fuma2 cooler was getting heated by the GPU exhaust. Not a huge deal, CPU temps went up like 5 degrees, nothing major, EXCEPT, the GPU was also scorching hot (like memory junction temps hitting 100 hot).

I tested everything I could, this fan at that speed, that fan at this speed, flipping fans (where it makes sense), and there was just no getting around it, the problem was that even with the massive dual fans at the top of this case, there was not enough exhaust room for that GPU to push the heat away from itself. The air cooling stack had to move out of the way.

So for the first time in history, I put an AIO on one of my personal PCs. I also switched the right side to exhaust. Immediately everything was working great. GPU and CPU are in the 70's at full load, with memory junction temps hitting 80 int he worst case scenarios.

But now the case was like a dust vacuum, massive negative pressure. So I got a new bracket to allow fans on the left to go in there. They don't really do anything to improve cooling, but they do increase positive pressure in the case, so now that left side is almost exclusively the intake, and the exhaust is the right side. The water cooler radiator is so massive (it's a 420 liquid freezer) that the gpu also exhausting through it basically does nothing and CPU stays cool.

Without water cooling here I would have to give up this case, which I was not prepared to do because to me desk space is the most important thing to have in a case. (it's also on a electric standing desk so I couldn't just mount the PC to the wall or something without having a wiring mess).

Surely there are other configurations where watercooling is just the only remaining option like this. I wouldn't ever bother with a water cooler otherwise though, for nearly everyone here air cooling is the way to go.