I’m not too familiar with the ACTs, but a friend of mine scored in the 1900s on the SATs and got a 35 on his ACT exam (an almost perfect score). I would take official practice tests from both (not a test from Kaplan or another test prep company), under timed conditions. See where you score higher and which structure you personally prefer. I found the SAT simpler but trickier, while the ACT seemed to be more straightforward but testing more difficult concepts. I would pick one test early on and focus on it rather than trying to score high on both tests. I think that’s a smarter strategy.
Good luck!