You can compare your scores on the practice tests by looking up the ACT on Wikipedia–they have a score conversion between the SAT and the ACT.

But to tell you the differences: ACT has a science reasoning section. ACT math is a little more advanced, since it tests all the way through trigonometry (but don’t worry–it will only have a couple of really advanced questions). ACT Writing section is optional (but you should do it anyway). ACT questions aren’t built to trick you, so the test is more straightforward than the SAT is. There is no guessing penalty on the ACT (but mind you, if you did guess everything, you’d still only end up in the 1st percentile, according to odds).

Both tests are standardized to similar groups of students, so your scores, when converted, shouldn’t be too different unless you are really good or really bad at writing or science reasoning.

I would recomend taking both though and using the one that you score highest on. Some schools with prefer one over the other anyhow. Schools on the east coast usually prefer the SAT.

They’re both pretty easy. Just make sure not to drink a lot of liquid before the test. I would have done tons better on my SAT if not for that