The "old" SAT penalized students for guessing, but that was changed to be like the ACT which has never penalized a student for guessing.
Both have a math test, a reading test, and an English test.
Both exams are approximately three hours long.
Both tests have an optional essay. This costs more and makes the test day almost one hour longer. The good news is less than 3% of the colleges in America require the optional essay score, so the vast majority of students do not need to do this.