Your school may be SAT-focused or it may be ACT-focused, and you may choose to go with that flow-that's absolutely sensible! Increasingly, students are given a choice between the two tests. We recommend against taking them both to see on which you have a higher score - that's too much testing. It's a good idea to compare your performance on the two, though. You should do a diagnostics test (a practice test, that is, with no studying in advance) for each, to get a benchmark score and to see whether you have a clear advantage taking one over the other. You can take the diagnostic tests at your school if they're offered, or for free through one of the following sources.
For the ACT, free diagnostics are offered by ArborBridge and RevolutionPrep (or any test prep company you like).
For the SAT, you can take a FREE practice diagnostic test through Khan Academy. Khan Academy is only test prep organization that has access to real practice questions written by the College Board (the College Board makes the SAT).
Another plus for Khan Academy is that the College Board links your PSAT scores directly to Khan Academy. Khan Academy (we mentioned that it's FREE, right?) then selects videos for you based on areas in which you've performed less well.
Once you've taken the diagnostics tests, chose the test you feel more comfortable with, and stick with it.
SAT and ACT: The Comparison
Why Take It - No difference between the reason to take them: colleges use the SAT and the ACT results for the same reason -to have data about the applicant.
SAT: Colleges use SAT scores for admissions and merit-based scholarships.
ACT: Colleges use ACT scores for admissions and merit-based scholarships.
Writing and Language
3 hours (without essay)
3 hours, 50 minutes (with essay)
2 hours, 55 minutes (without essay)
3 hours, 40 minutes (with essay)
SAT: 5 reading passages
ACT: 4 reading passages
ACT: 1 science section testing your critical thinking skills (not your specific science knowledge)
Algebra I & II
Geometry, Trigonometry and Data Analysis
Algebra I & II
Geometry and Trigonometry
SAT: Some math questions don't allow you to use a calculator.
ACT: You can use a calculator on all math questions.
SAT: Optional. The essay will test your comprehension of a source text.
ACT: Optional. The essay will test how well you evaluate and analyze complex issues.
How It's Scored
SAT: Scored on a scale of 400–1600
ACT: Scored on a scale of 1–36
2017-18 SAT registration and dates: https://collegereadiness.collegeboard.org/sat/register/dates-deadlines
2017-18 ACT registration and dates: http://www.act.org/content/act/en/products-and-services/the-act/registration.html