Students often ask me which standardized test will look better to a college or university in the United States. The simple answer is: It doesn't matter! Here are the differences between the SAT and the ACT.
The ACT has traditionally been considered to be an achievement test, measuring what a student has learned in school. The SAT has traditionally been considered to be an aptitude test, measuring reasoning and verbal abilities. While the SAT used to be the more widely used test, many students have been taking the ACT in recent years. Seeing this shift (and watching their numbers drop) has forced College Board, the organization that administers the SAT, to shift more toward the SAT model. This will begin in the spring of 2016.
The ACT has up to 5 components: English, Math, Reading, Science, and an optional Writing Test. The SAT currently has 3 components: Critical Reading, Math, and a required Writing Test.
The SAT penalizes you for wrong answers, so guessing is discouraged. The ACT is scored based on the number of correct answers without a penalty for guessing.
While I don't think there is a right test or a wrong test, I think every student should take practice ACT and SAT tests to determine which one they feel more comfortable taking. Most students will naturally gravitate toward one test or the other.