Take one released old ACT under test conditions (time limits, etc.), then grade it to determine which type of questions you have the most difficulty on, in order to determine where to focus preparation on.
But also, take one released old SAT under test conditions and grade it to determine if you do significantly better on the SAT than the ACT. You can see the concordance tables at https://www.act.org/content/dam/act/unsecured/documents/ACT-SAT-Concordance-Tables.pdf . For example, 24 ACT is like 1160-1190 SAT, so if your practice SAT with no preparation is substantially higher than 1190 (or the SAT equivalent of your practice ACT score if you also did that), then you may want to focus on preparing for and taking the SAT instead of the ACT.
The ACT has a reputation of having easier questions, but more time pressure, than the SAT. Based on the above, you could have a good likelihood of doing better on the SAT.