I think you won’t know until she’s tried both. I am also much better at math and science in school, but I found that I didn’t like the ACT math or science section because it was basically geometry/trig and more reading (all things I hate). I took the ACT 3 times to end with a 34, but I just got back my school SAT and I got a 1560. I would say that it would be best to let her try both tests once, and see which one is more naturally comfortable with her.