One difference is the direct testing of vocabulary on the SAT vs. Science reasoning on the ACT.

What's the Difference
Between the ACT and SAT?

Always check with the colleges to which you're applying. Almost all (including the Ivy's) accept both equally.

Given that your standardized test score is arguably the second most important component of your college application, it’s essential to think about which test best shows what you can do. Beyond starting early, what matters is focusing your efforts where you’re most comfortable and most likely to be successful.

So, how are they different, and what factors might make one or the other a better choice? Here are some general thoughts that you’ll want to consider.

  1. Most students find the ACT more straightforward than (not as "tricky" as) the SAT. However, some students feel more pressure with time on the ACT, as there are more questions to answer in the same amount of time.
    Not including the essay, the average SAT section requires you to answer 19 questions in about 25 minutes; on the ACT, it’s 54 questions in about 45 minutes.
  2. The ACT is very predictable — the same four (or five) sections in the same order. The SAT jumps around between Critical Reading, Math, and Writing sections.
    How’s your focus? Do you get into a groove and prefer to keep focused on one topic until it’s finished (ACT). Or, do you like changing it up and prefer shorter "tests" with breaks in-between (SAT)? Are you more comfortable when you know what’s coming ahead of time (ACT), or are you okay with surprises (SAT)?
  3. The Math questions on the SAT tend to be preferred by those students who are "clever," who enjoy finding shortcuts and tricks. Success requires logical thinking and careful reading. While the ACT tests higher-level concepts (trigonometry), the questions tend to be easier to read and understand.
    In your math classes, do you tend to see the answer quickly and dislike having to show all of your work just to prove that you know what you're doing (SAT), or do you like straightforward questions that don't try to trick you (ACT)?
  4. The ACT’s Reading section is time demanding, but the answers are right in the passages. In addition, there’s limited emphasis on vocabulary. The SAT sections directly test higher-level vocabulary; the reading passage questions rely more on inference and the answer choices tend to be less clearly different from one another.
    How’s your vocabulary? Not great? (ACT). Strong? (SAT). In both cases, you definitely need to be a very good reader.
  5. The SAT Writing sections focus on grammar. The ACT English section focuses on editing skills, in particular, punctuation.
    Beyond, grammar (SAT) versus punctuation (ACT), don’t forget that the SAT Writing score includes the essay, whereas the ACT English score, and thus the Composite Score, does not.
  6. The ACT has a Science section. Actual science knowledge is not tested; instead, science-based reading passages test your ability to interpret, analyze, and evaluate the information presented.
    You don’t need to memorize science facts, so how are you at reading and interpreting scientific information? Consider the ACT if this is something you enjoy and are good at.