What are the advantages of the ACT over the SAT

The ACT is a subject-area test. it tests your knowledge on a variety of subjects (look at the test booklet to see what subjects are covered). The SAT has some content knowledge such as vocabulary, high school core math concepts for a college-bound student (not trig or analyt). It tests your application of that knowledge to a variety of situations. You could say it is more of a "thinking skills" test. You may wish to take them both to see how well you do. If you must take one or the other, and you are a good memorizer of information in your core high school classes, take the ACT. If you are good at application of learned information, take the SAT.