OK here is the simple truths.
A2A units are more suitable for street use, they have a greater simplicty, greater thermal efficiency at high speeds, greater reliability, lower matiene, and lower cost.
A2W units generally will have better themermal efficiency at low speeds, better throttle response with maf equiped cars, lower boost pressure loss, and less compressor surge.
Also as for the heat transefer coeffiecents well water wins, by 14:1 that of alumimun. If you really want to be cool and use your brain, get a copper radiatior core with your A2W intercooler, and you can thank me later.
You also will have to take a lot of consideration into the size of the resevior and or front cooler (which imo is a must), so the bigger the resivoir , the greater mass of water, which equals the greater thermal inertia ( cooler charge ). As for the front cooler aka radiatior, well its not too important but its usefull. At the start of a boost run, the entire system will be around ambient temperature, as the boost rises, heating the water in the main core, that heated water must get to the front core (radiator) before it has any temperature difference with which to drive the heat out. On avg the time delay is said to be 7-8 seconds, depending on resivoir. So the radiator does most of its work out of boost, because the temperature difference between the water and the radiator is small comparied to that of the boost charge, and water, and it takes a lot more time to cool water down then, to heat it up.
All in all Scott they both work well, for what they ar designed, A2A have proublems with pressure loss, which increases the exhuast manifold pressure, which negates some power you gained from intercooling. (I can PM you some equations to size the A2A unit for you and get you as close to zero resistence as possible). A2W units weigh more, more complex, and you have to figure out a way to keep the water cool. So thats about all I can ramble off right now.
Oh air/fuel ratio > intercooling device = importance.