The short answer is: you should take BOTH! Colleges accept either test score, and they don't have a preference between the SAT and the ACT.
Each test has advantages and disadvantages. The SAT contains fewer overall questions and gives you more time to complete each question, but the problems tend to be more ambiguous and purposely misleading. The ACT, on the other hand, requires significantly greater time management, but the questions are much more straightforward.
Oftentimes, a student is better suited to one of the tests. His ACT score, for instance, might be considerably higher than the equivalent score on the SAT. The best way to get your highest score is to take both tests a couple of times.