Colleges compare the two basically by which one they care about. Generally, the East and West coast schools prefer the SAT. Midwest schools prefer the ACT, gnerally. I'm not too sure they do the crossover score thing, but they might. From everything I've heard though, just look at the schools you're applying to, check their class bios, and see which scores they report averages for. If they report SAT, take the SAT. If they report ACT, take the ACT. Simple as that.