I assume you already know these two exams, so how to pick which one to take?
The Best Option
(only apply to those who still have a lot of time to prep for the test ex. Freshmen, Sophomore, or Junior who has nothing else to do):
Take both test. Do the practice tests (only the ones that are very close to real one) and see which one you score higher. And then, choose the one that you scored higher!
SAT -- if you are more into English
ACT-- if you are more comfortable with math and science
In addition, because SAT is more of a "reasoning" test that tests your ability, it is more tricky to most students. And for ACT, it is more a straight forward test that test you on certain required topics. But in conclusion, you have to study for it before you take the actual exam. You have no idea how many students mess with these serious exams and ruin their chances of getting into dream colleges.
If you have any questions related to this topic, feel free to ask me!