I think it’s preferable to take practice tests of both to determine which best fits you as a student and then focus in that one test. Test question type and timings vary between the two - some prefer one, some prefer the other. Many find neither preferable.
If you’ve already taken one and determine it’s not the better test for you, then it would make sense to switch and end up taking both.
Unlike what some students seem to believe, there is no “bonus” to sending a “back up” score from another test type to “prove” your score is legitimate.
Sending a 1550 SAT and 35 ACT will not be better than just sending a 1550 SAT.
The ACT Science section is mostly about data analysis and interpretation. It will not be treated as a substitute for an AP science course/exam.