Versus the SAT, the ACT generally presents questions in a less complex manner but requires students to know more advanced concepts and work more quickly.
However, our experience working with thousands of students over the last 18 years is that fewer than 5% of students will score significantly different on the SAT vs the ACT. After the SAT remade itself in 2016, the ACT and SAT are incredibly similar and the overwhelming majority of students will score equivalently.
Both the SAT and the ACT test core concepts from Math, English, and Data Analysis. They are both peer-normed tests (you’re scored in comparison to your classmates). They both have an optional essay.
Top colleges like Harvard, Princeton, and Stanford could not care less which test you take (so long as you do well); both the ACT are nearly universally accepted at US colleges. The ACT and SAT go back and forth between which exam is technically more popular when counted by completed tests; depending on the year, the ACT and the SAT each test between 1.7 and 2.2 million college-bound seniors.