Many students are asking me if they should take both the SAT and ACT! There are some pretty stellar reasons to forego that option.
You should definitely not take both the SAT and ACT, unless you've found out after taking one or the other that the school you're interested in only takes one of them. Most schools accept either set of scores.
You should choose either the ACT or SAT and not both because it's easier for you to master one exam than two. Learn the testing tips for the SAT and take that. Study the ACT strategies and get really, really good at them. Practice the questions for either one of the tests and master those questions! Each test is drastically different and requires different testing strategies. If you take both tests, you're reducing the overall score you could get on either one.
So how do you decide which to take? You could take this brief ACT vs. SAT 10-question quiz to see which fits you best, or take a gander at the practice questions in your favorite test prep book to see which one seems to suit your style.
Because the exams vary widely and require considerable prep time to master, it's in your best interest to just take one or the other, choosing wisely before you test.