In a purely scientific sense, an air-water IC is less efficient than air-air because it uses two heat exchangers instead of one, and you lose efficiency with each exchange. But it has a big advantage in that you can potentially cool the air charge to below the ambient air temperature (with ice in the water tank), which you can't do with an air-air exchanger. Plus you have more flexibility in the size and location of the exchangers. For every day street driving, I think I'd stick with air-air, but for all out racing the air-water would probably be better.
BTW, when is the next water show? Somebody should put a sign...