The ACT is written in Iowa and the SAT in New Jersey. Philosophically, they reflect the regions they are from. The ACT is more straightforward, following closely to high school math in the math portion of the test. In reading, there’s less interpreting and more just remembering what you’ve read. The SAT requires more problem solving techniques in math and more “reading between the lines” in the verbal sections. The ACT also includes a science section. There is actually very little science knowledge required to be successful in that section; it is mostly interpreting graphs and charts. The ACT has longer sections but fewer sections. The overall length of both tests is about the same. Although both tests are time pressured, demanding students to work quickly, the ACT is more challenging to finish in the allotted time.
Although preparation for one test will help for the other, they are different enough that they need to be prepared for separately. High schools provide booklets for each test, usually in the counseling offices, that include an actual complete test that can be scored. Then students can see which test is better for them. You can compare scores at online sites that will tell you how SAT and ACT scores correlate.
Both tests have an essay although in the SAT it is part of the writing score and in the ACT it is a separate score.