Intercoolers are better from what Ive heard because you have cool air hitting it when your moving which intern keeps you car cooler when driving any distance.
Where as a aftercooler it still cools it down but the way to get it very cold is to add ice.
I dont know if there is any proof that one is actually better than the other at the track. But I have been told if you do any racing what soever on the street intercooler is the way to go.
I just picked up a Paxton IC and it should be put on some time next week. Tim @ MPH reccomended it to me saying that its good for about 600-800 rwhp, which is more than Ill ever need..... hopefuly.