I suggest the SATs. More common, and in several ways, easier to use to get into college. You can take it three times and many colleges will as Kraken said, take your highest scores of each category. Percentiles are often pretty low for the SAT, too. Think about it more this way: if you do really well in your final exams, or in state testing, maybe the ACT is for you. If you prefer to just take a reading/math test with a bit of a 'logic' component, the SAT. The thing with the SAT is really just strategy. If you know the techniques, then you can see through pretty easy to the answers.
Also, I'm not sure about the ACT, but the SAT doesn't require you to answer every question. I really like that fact about it.