National Public Radio gives some advice on who is best suited for which test. I have some better advice, though: take both tests during your junior year and determine if you’re stronger in one or the other. If you are stronger on one then you can focus on your stronger test over the summer and re-take it one or two times the fall of your senior year. If neither stands out, I recommend taking both the SAT and ACT again during your senior year. More from NPR.