While colleges will accept either one and say they don't care which you submit, a lot of folks on the East and West Coasts consider the SAT to be superior. It doesn't really matter but the ACT is an achievement test, while the SAT used to be more of an aptitude test. Unfortunately, the SAT is now closer to an achievement test (making it easier, at least in my opinion).
In the end, my eldest had very similar scores on the two tests. She did some prep for the SAT but took the ACT after trying just one practice test - she will tell you the ACT is easier.
