The discussion of air to water and air to air intercoolers is currently going on in the GTM thread but I did not want to jump in on that thread so I will post our reasoning for air to water in this thread...
Please feel free to do any amount of research on this as you like. The results will always be the same. The bottom line is, water is a more efficient cooling method than air. Water is four times more efficient at dissipating heat than air. It's thermodynamics, not some STILLEN voodoo...It just is.
This is a pretty good explanation of the benefits of air to water intercooling:
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One of the other benefits of the air to water intercooling is that the heat exchanger in front does not consume all of the airflow to other vital coolers such as engine oil, power steering, automatic transmission fluid, a/c condenser.
yes, there is a bit more weight in the air to water intercoolers because you do have to account for the weight of the water and additional pumps, but we're talking fractions of a percent when comparing the overall power to weight ratio of the entire vehicle.
The other thing to consider is that the majority of O.E. manufacturer's with supercharged vehicle's use air to water:
Ford
Ford GT Supercar
Lightning
Ford GT500
Ford Mustang Cobra
GM
Corvette ZR-1
Cobalt SS
Toyota
TRD supercharger for the 07+ Tundra
TRD supercharger for the older Tacomas
Aftermarket supercharger manufacturers who use air to water intercooling:
Edelbrock
Vortech
Magnuson
Whipple
Kenne Bell
Saleen
Steeda
As everyone has said...Air to air is less expensive, if it was better, wouldn't major manufacturer's use it in their supercharged applications?