There are some significant differences between the SAT and the ACT. Even so, both exams share the same purpose of attempting to measure what one learned in all 12 grades in school.
The presentation of both tests is very different even though the content tested is very similar in some sections. The ACT has a Science section, and many students who do not like science automatically believe they should not take the ACT. Many students are falsely deterred from taking the ACT for it's Science section, but it's not so simple. The ACT tends to be a little more straight forward but requires a student to be quick. The SAT can take on a trickier approach but allows a little more forgiveness in terms of timing.
Truly, the only way to know which test a student should focus on is taking both tests at least once. Since the scoring systems don't use the same point system, turn the scores into percents. Many students who originally planned to avoid the ACT because of its Science section discover that they score higher on the ACT than the SAT.
KEEP IN MIND: Many students start studying for the PSAT (Pre-SAT) in elementary school. Duke University has a program that allows students to take the PSAT in the 7th grade. The College Board also has a PSAT 8/9 for 8th and 9th graders and PSAT 10 for 10th graders. Even if a student decides to focus on the ACT, all PSAT & SAT preparation
will help ACT scores. The content in both tests are very similar (except Science section), even if the presentation is different.
See the SAT and ACT videos to find out how and where to go to take your own free practice tests.