One difference is the direct testing of vocabulary on the SAT vs. Science reasoning on the ACT.
Always check with the colleges to which you're applying. Almost all (including the Ivy's) accept both equally.
Given that your standardized test score is arguably the second most important component of your college application, it’s essential to think about which test best shows what you can do. Beyond starting early, what matters is focusing your efforts where you’re most comfortable and most likely to be successful.
So, how are they different, and what factors might make one or the other a better choice? Here are some general thoughts that you’ll want to consider.
Not including the essay, the average SAT section requires you to answer 19 questions in about 25 minutes; on the ACT, it’s 54 questions in about 45 minutes.
How’s your focus? Do you get into a groove and prefer to keep focused on one topic until it’s finished (ACT). Or, do you like changing it up and prefer shorter "tests" with breaks in-between (SAT)? Are you more comfortable when you know what’s coming ahead of time (ACT), or are you okay with surprises (SAT)?
In your math classes, do you tend to see the answer quickly and dislike having to show all of your work just to prove that you know what you're doing (SAT), or do you like straightforward questions that don't try to trick you (ACT)?
How’s your vocabulary? Not great? (ACT). Strong? (SAT). In both cases, you definitely need to be a very good reader.
Beyond, grammar (SAT) versus punctuation (ACT), don’t forget that the SAT Writing score includes the essay, whereas the ACT English score, and thus the Composite Score, does not.
You don’t need to memorize science facts, so how are you at reading and interpreting scientific information? Consider the ACT if this is something you enjoy and are good at.