The SAT is taken more on the East and West coasts, but the ACT is ahead with 1.9m test takers compared to 1.7m SAT test takers in 2015.
Since 2007, all four-year colleges and universities in the United States that require a test as part of an application for admission will accept either the SAT or ACT.
What may happen is that schools will require SAT2 scores which doesn't have an equivalent from the ACT. In that case if you need SAT2 scores you might as well also take the SAT, but in all other cases you might as well flip a coin.