I tutor the verbal sections of the tests, but I know a lot about the math sections of both. The two tests are NOT the same. In particular, SAT math is nowhere nearly as straightforward as ACT math, and has a non-calculator section. The SAT is, however, much more generous on time. The ACT math section definitely goes to a higher level of math, but the time constraint is a killer. Many kids prefer ACT math to SAT math because it’s easier to understand.
My son got good scores in the math section on both tests, with a 790 on SAT math, but a 31 in ACT math. He thought SAT math was a lot easier. He rushed to complete ACT math. This is the complaint I most often hear about ACT math. Again though, a lot of kids find SAT math tricky because much of it is word-based.
I don’t hold much store by Pre ACT results. They are taken when kids are still sophomores and haven’t had the level of math the actual ACT goes to, though that’s probably taken into account on the pre ACT, but not on a practice test. For the Pre ACT in general, I find that it rarely gives an indication of an actual ACT taken as a junior. The PSAT does a little better as an indicator.
The vast majority of students take the pre tests with no preparation at all. Any kid who does a little prep beforehand is going to get a better result on an actual test. I never recommend kids take an official test as a practice. Do a practice test of each at home (or better, go to the library), for free, without the stress, money, and time commitment.
In my experience, kids typically don’t have equal scores on both tests. IOW, kids most often have a better score on one test or another. If the student takes a practice test of both, prefers one over the other, prepares, and gets a good score on an official test, I see no need to take the alternative. However, there is probably no harm in it, IF the parents can afford to pay, the kid has time, can handle the stress, and perhaps isn’t applying to a college that requires all test scores.