I remember hearing on CC that a few schools !!!specifically state!!! that they prefer the ACT, but they are few and far in between. In our case, let's just assume they don't exist.
Take the test you'll get a better score on. (some schools require SAT II's make sure to look into that)
Red book for ACT/Blue book for SAT (do the third/forth practice exam in each book) compare your results. i max out 1600 on SAT... lol I got a 28 on my ACT
If you look around the ACT/SAT forums there have been many questions asked on the differences between each test that you can look into.