The New York Times has an interesting piece on the merits of taking the SAT versus the ACT. When I was applying to school I took both of them and did slightly better on the ACT, but not significantly. I also did not prepare for either test, taking them straight up with no courses or books that taught me how to do so. My recommendation for those applying to college right now would be to just take both. You get to choose which scores to submit to which schools anyways, so if you do poorly on one you can always submit the better of the two. It might kill an extra Saturday morning, but it'd be worth it. If you are doing preparation though, then it's probably best to just do and focus on one of the tests.