It's really not a big difference either way, air coolers have improved with the use of heatpipes and water coolers dropped the performance they were associated with when they went to compact aio's bringing them pretty even. That and the fact that modern cpu's don't put out the heat they once did. The only way to get better cooling on older overclocked cpu's with lesser performing air coolers (aka a hunk of metal and a fan) was to go water cooling. Current air coolers are very efficient and many use high quality fans capable of cooling at lower sound levels than most aio's. The fact they have less moving parts (just a fan or two) and no water to potentially leak makes them pretty reliable and safe in comparison.
Anymore water cooling is more done for the 'cool factor' or as a hobby, not a necessity. Some will argue that aio's don't usually leak which is true. The fact remains that air coolers can't leak, period. Even if an aio were able to provide me with 3-5c better cooling, I personally don't find it worth the risk of the entire system.