I was an SAT and ACT tutor for years, and I’ve had to have this conversation with students and their parents many times.
I mostly agree with PnL’s statement that the ACT is “more based on actual stuff learned in high school” and the SAT “is all about test-taking skills and strategies.” But let me elaborate.
Different students tend to do better on one or the other, depending on their skill sets.
The ACT consists of 4 types of sections:
English: The English questions are 75 multiple choice grammar questions. Similar to those on the Writing section of the SAT. The main difference is that the ACT is allowed to ask questions dealing with punctuation; the SAT does not do punctuation. The ACT English section consists of 5 passages with grammatical mistakes throughout them. The mistakes are underlined and numbered, and you need to choose what phrase best corrects the grammatical mistake. The grammar mistakes tested on this section are very easy to learn. I often brought students who originally scored 50/75 to near perfection.
Math: The Math on the ACT deals with more advanced math than does the SAT, but the questions themselves are easier (if that makes any sense). ACT includes everything up to and including Trig, but there are only about 4 (of 60) questions that deal with Trig, and it is very basic Trig that is quite easy to learn. The main difference between SAT math and ACT math is that the questions on the ACT math section are akin to questions you would see on an actual math test in high school. They are pretty straight forward and test simply your ability to carry out the math procedures you learned in Algebra 1, 2, Geometry, and Trig. (The SAT makes you use your ability and apply it to problems that are more logic based, rather than simply performing a particular type of problem.)
Reading: There are 4 passages, each with ten questions. The biggest obstacle here is time. You only have 35 minutes to complete the 40 questions. This can be accomplished with practice. Biggest difference here: no vocab, as there is on the SAT.
Science: You mentioned that you know biology but not chemistry. That doesn’t matter. The Science section of the ACT does not test your knowledge of science. It tests your ability to understand science. You are presented with several “passages” – usually descriptions of experiments or studies. Given the data they provide, you are expected to make inferences about what would happen if a certain variable was changed, for instance.
There is also an (optional) essay, and it is much easier than the SAT essay because you are not expected to use academic examples from history and literature. It is a 5-paragraph essay on a topic like, “Should students be required to maintain a C average in order to receive a driver’s license?”
The SAT consists of 3 types of sections:
Critical Reading: Passages with questions, but also sentence completions – which test your vocabulary.
Math: Only uses concepts from Algebra 1 and Geometry (maybe some Algebra 2) – but does not test them as directly as the ACT. You aren’t simply asked to complete a specific problem. You need to think a little more with the SAT math and apply what you know to the question.
Writing: The writing score is 70% multiple-choice grammar questions, 30% essay. The SAT grammar questions are largely similar to those on the ACT, but they’re not concerned with your knowledge of punctuation usage.
The essay on the SAT is “more difficult” because it requires that your body paragraphs are examples from history, literature, or some other scholarly thing.
Personally, if you’re going to work at preparing for one or the other, I think the ACT is easier to “master.” I never get any ACT questions wrong, but I still get a few SAT questions wrong from time to time.