Easy answer: Both, or neither!
Students often ask if they should take the SAT or the ACT. The answer is, they should take both tests one time, then take the test they are most comfortable with a second time.
The tests are slightly different, and one test will stand out for every student. I counsel students to take each test one time. Then prep for the test they prefer — either private one-on-one tutoring, private group tutoring, or free online prep. Free SAT tutoring online is hands down better than free ACT tutoring. SAT’s free online Kahn Academy is excellent for students who are self-starters, and learn well on their own.
Students who have conducted effective prep should expect a 5 - 10% increase in their score on their second attempt. Generally, students who test a third time do no better than the second time, and so I do not recommend a third sitting.
All colleges accept both SAT and ACT, and an increasing number of colleges do not require standardized test scores at all. If a strong student tests poorly, a “test optional” college can be a good option.