The ACT is an achievement test that focuses on school curriculum-based skills — English, math, reading, and science. The SAT is more of a reasoning test that changes very subtly from one test to the next but still includes math, writing (English grammar), and critical reading; it does not contain a science component.
The similarities between SAT and ACT can lead to confusion about which way you should go. Both SAT and ACT are nationally accepted standardized tests for admission to educational institutions in the U.S.A.
All U.S. universities and colleges accept both SAT and ACT, so there is no question of one examination having more value than the other.