Depends on what kind of overclock your going for and what kind of case you're getting.
Water AIO's are very slick but if you're not getting a dual fan radiator you're cooling isn't going to be much different than air cooling. They also have a more limited life span than air simply because the pumps have a live span. But if you get a good part in there, they'll out perform big air coolers. They also can give you better cooling if your case is on the small side, where a big air cooler can't be installed.
Air cooling, if you get a good one, can often compete with water cooling and do it cheaper. But they're MUCH bigger for the performance and can give you issues with your RAM selection. Big air coolers typically over hang the RAM slots...