This is an excellent question. Maybe the single most important question. The answer is "it depends".
But the first big fact to hold onto is that ALL COLLEGES and UNIVERSITIES accept BOTH the SAT and the ACT. They are treated exactly the same. Schools do NOT prefer one over the other.
Next, BOTH exams are offered at least six (6) times each year September thru June.
Next, BOTH exams are now very similar in style and content.
Finally, you can get real live previously administered SAT and ACT exams. See below.
So you have options. But how to decide? Here is the trick you must follow no matter what--this is Non-Negotiable.
Now self-administer both exams and see how you do. If you find that you prefer one exam over the other or that you did better on one of them--well you've now answered your question.
IMPORTANT NOTE: Do NOT under any circumstance use a practice exam offered by a test prep company like Princeton or Kaplan or any of the other 100 SAT/ACT test prep companies. In order to evaluate the SAT and ACT you must use real, officially released exams created by the companies that make the exams. Fortunately it is rather easy to get your hands on actual SAT's and ACT's from previous administrations.
Two OPTIONS: