Since both tests are accepted by colleges and universities in the US, you need to determine which test is going to make you more competitive in the eyes of admissions officers.
The newly released SAT is nearly identical to the ACT. They both test your English grammar, reading, and math skills, but there are some differences…
The SAT is considered to be easier because it does not include a science section and test-takers are given more time per question - but it is very important you remember these tests are graded on curve. This means that your score is determined by comparing your performance against that of the other test takers. So, while the SAT may be “easier”, it is easier for everyone who takes it. And since the ACT is harder for everyone who takes it, this is something you can use to your advantage.
If you are able to work quickly and excel in science reasoning, then taking the ACT will probably give you a huge advantage over your competition. If, on the other hand, you prefer to take your time and think through your answers, the SAT is likely the better option.
So what now? Should you take the ACT or the SAT? Start by taking complete, timed practice tests for both of them. Then consider which test you prefer and perform better on.