According to the ACT and SAT websites, all schools accept both tests. However, some schools prefer that students take one test over the other, and a few admissions offices prefer that students take both tests. Consult the admissions office at the school of your choice to see if they have a preference about which test you take.
The ACT has a science section, while the SAT does not, and the ACT has more challenging math problems than the SAT. The SAT, by contrast, has a stronger emphasis on vocabulary. If you're good at math and science but struggle with vocabulary, the ACT might be a better choice. The ACT writing section is optional, although many schools require that students take this section. If your school doesn't require the writing section and you have weak writing skills, the ACT may be a better choice.
The SAT has more sections than the ACT, while the ACT is broken down into longer sections -- each dedicated to a particular domain of knowledge. If you're easily distracted or need frequent breaks, the SAT might be a better choice. But if you like being able to structure your test-taking time in a way that works best for you and don't tend to lose focus during long periods of testing, the ACT could be the best option.
If you're not sure which test to take, it might be best to take both tests. While SAT questions tend to be indirect, ACT questions are often shorter and simpler, and each approach has its advantages depending on your test-taking skills. If you find that you score poorly on one test, you could get a higher score on the other test. Contact your college's admissions office to see if it accepts the higher score or requires that you submit both scores.