The ACT is more knowledge-based, while the SAT is more reasoning-based. DC20 didn't take it much earlier than usual, but his scores were higher on the SAT (2E, though, so some reading issues might have happened with the ACT). DC17 scored better on the ACT than the SAT, but she's more of a high-achiever with a lot of effort.
A very long time ago, I took both as young pre-teen. My SAT scores were higher than my ACT scores, although they were somewhat similar. I wasn't accelerated in school at that point, so I struggled a bit with the purely knowledge-based questions in the writing and math sections. That would be something she could look over before the exam, though. I believe there are review materials for the ACT these days.
