All else being equal, go for the largest power supply you can get, and make sure it's a good brand. Your computer will use only the power it draws from such a supply, which means that if you are drawing 250 watts from a 500-watt supply, the power supply will still be using only about 250 watts.

This margin is good to have when you add things to the system, and costs you only a bit more up front. A power supply being forced past its maximum is not a good thing.

Any good-quality power supply should have its own cooling adequate to its maximum rating. Auxiliary cooling shouldn't be necessary.

The only reason I can see for a liquid cooling system might be noise issues (bigger or faster fans make more noise, and it does add up). I'm leery of them, though.