All states, and some international sites, offer both the SAT and the ACT. The short answer is: take the one you’re better at, and take it as many times as you can, while preparing a bit each time.
All colleges and universities take either the SAT or ACT. So focus on the one you prefer. The SAT is trickier, the ACT is more time pressured. The ACT may seem more user friendly to you, as it was designed to follow your academic training a bit more closely. But both the SAT and ACT take simple stuff you learned in school, then dress it up to look much harder than it is. That’s the key to understanding both tests.
There’s no harm in taking both tests, and it can even help by giving you more chances at a great score. But most people only have so much time to devote to test prep, so you probably want to focus on one or the other. For what it’s worth, I find most of my students prefer the ACT and seem to perform more consistently well on it. So I recommend that all of my students at least try it, even if they are in a predominantly SAT state and all their friends take only the SAT. There’s no harm (and potentially a lot of good) in being different!