The decision whether to take SAT or ACT is a tricky one. A few different factors need to be taken into consideration.
You don't have to take both tests. Every college accepts both.
If you do well on either SAT or ACT scores are not the only factor. Grades, courses selected, extracurriculars, etc. matter a lot.
ACT fits way more questions in the allocated time. If you struggle with time management, take the SAT.
ACT has a science section, SAT doesn't. ACT science section doesn't test your knowledge of science facts, but rather tests whether you can analyze complex scientific data (e.g. graphs, scientific articles) to answer questions.
ACT tests higher level math concepts.
Other differences like relative weight of math section, and how reading comprehension section is structured.
To find out which test suites you, take a practice test for both, and see for yourself!