For most students considering applying to college this is a big question. Across the nation students take the PSAT in October of Junior year. So they are familiar with its test format and question types. But not many students get a "preview" of the ACT.
The SAT or the ACT are required by most colleges as part of the application materials. Your scores from these standardized tests are used by the Admissions Committees to determine your acceptance to their college. So you want to be sure you are taking the test that fits you best and that you can score the highest on.
Luckily both the SAT and the ACT provide a comparison of the 2 tests on their websites. https://collegereadiness.collegeboard.org/sat/inside-the-test/compare-new-sat-act https://www.act.org/content/act/en/products-and-services/the-act/scores/act-vs-sat.html
This chart from the SAT compares the test format between the two tests including how much time is allowed per question.
This chart from the Princeton Review also compares format and a few more details of the SAT vs the ACT. https://www.princetonreview.com/college/sat-act
This chart from Blog PrepScholars also compares the content of the two tests. And if you want a more detailed assessment of 11 key differences between the SAT and the ACT then check out the whole blog post here https://collegereadiness.collegeboard.org/sat/practice
Ultimately the best way to decide which test fits you better is to take a practice test or do practice problems from each test. Luckily both the SAT and the ACT provide free practice tests and practice problems on their websites.
https://collegereadiness.collegeboard.org/sat/practice
https://www.act.org/content/act/en/products-and-services/the-act/test-preparation.html
Once you decide which test is your test ( SAT or ACT), then focus all of your test prep on that test. And after you have taken an actual test, now both SAT and ACT offer free personalized practice based on your actual test results. No more wasting time practicing problems in an area you did well on. Your practice is geared to the areas you need to improve to help you boost your score the next time you take the test!
https://www.khanacademy.org/sat?utm_source=cbcta-satprl-spring-2019&utm_medium=cb418-cb&utm_campaign=practice
https://academy.act.org/?_ga=2.168918568.889204213.1556548415-2101637145.1527706117
Hopefully this information will help you take control of your college access journey!