The best way to decide is to take a timed full-length practice test under testing conditions of each type. You can take these practice tests on the College Board SAT website or ACT’s website.
Then, use the most recent ACT-SAT Concordance Tables to see which you scored higher on.
Don’t take the real test until you get the score you hope to on the practice tests. Colleges are not impressed when they see the student taking the test a lot of times (you could be doing other things more productive like volunteer work, sports, part-time job, personal passion project, etc….)
Colleges have different requirements, like "Test Blind," "Test Optional," or "Test Required."
“Test blind”: even if you submit your SAT or ACT scores, the college will not look at them.
“Test optional”: you are not required to submit SAT or ACT scores to be considered for admission to a particular school. However, you should not submit them if they are not within the target range for acceptances for that college. Test Optional colleges make an admissions decision based on grades, essays, extracurricular activities, and achievements. If you have strong scores for that school, submit them. However:
Many test-optional colleges look at test scores when awarding merit scholarships, so not submitting them might put you at a disadvantage. Be sure to confirm all scholarship requirements with the college.
Some require test scores for out-of-state students or for students pursuing certain majors
Some may determine your test-optional eligibility using other factors.
Some may ask for additional materials in lieu of test scores, such as samples of your academic work, scientific research, or additional recommendation letters.
"Test Required"
MIT (all SAT or ACT scores must be submitted, even if taken multiple times)
Georgetown
Brown University
California Institute of Technology (CalTech)
Cornell University
Dartmouth College
Harvard University
Johns Hopkins University
Stanford University
Yale University
Purdue University
University of Pennsylvania
University of Florida, Florida State, New College of Florida, Central Florida, Florida International
Georgia Institute of Technology, University of Georgia
University of Tennessee - Knoxville
Gallaudet University
Naval Academy, West Point, Air Force Academy
Similarities between the ACTs and SATs
Both are nationally recognized standardized tests and common requirements for U.S. schools.
Each measures students’ proficiency in various critical skill areas, like problem solving and reading comprehension, that are necessary for college success.
Neither penalize for incorrect answers. If you don’t know the answer, GUESS SOMETHING.