Shannon's answer is right on point. The other big question is whether on not to take the written essay portion.
It's true that many competitive schools no longer require it, but that doesn't mean they don't want you to do it. If you are applying to STEM programs and/or writing is painful for you, then you may skip it. However, if you think you might major in English, Journalism, History ... well, you should take it.
One point to note: If you take the SAT without the writing portion and later decide you want to add it, you will have to retake the entire test.
I would add that the written essay is quite different on the two tests. The ACT essay will ask you to state your opinion on the topic provided and back it up. The SAT requires a rhetorical essay in which, instead of taking sides, you examine how the author of a provided excerpt builds his or her or argument. There's a definite art to scoring well on this; I'd be happy to teach you!