How do these tests differ? Both are widely accepted college admission exams. The SAT is more familar to people on the West and East coasts, while the ACT is the most commonly taken test in the the Midwest and the South.
The SAT is made up of math, critical reading and writing sections (including a mandatory essay). The ACT consists of English, math, science reasoning and reading sections (as well as an optional essay).
The SAT is a slightly longer test and tends to have trickier questions. The ACT is slightly shorter, but gives the student less time to answer its more straightforward questions.
The SAT has a wrong answer penalty that the ACT does not include. The SAT provides five answer choices for each question while the ACT provides four. Both allow students to choose which tests to send on to colleges- this is called Score Choice. There are no specific vocabulary questions on the ACT as there are on the SAT, but there is a science reasoning section which does not appear on the SAT. The science reasoning section does not require any prior knowledge; it is in many ways a reading comprehension section. A few trig questions appear on the ACT, questions through algebra 2 appear on the SAT.
Which test do I take? First, check the colleges you're applying to to make sure that they all accept both tests. All four year colleges should accept either test, but double check to be absolutely certain. At that point students should take a practice test of each format and see if they do better on one than the other. If it proves impossible to take two complete tests, consider taking half of each (every other question in every section) or at least look over each test to see what kinds of questions they each ask and decide which seems most comfortable to you. It may sound as if the ACT is easier than the SAT, but that is not necessarily so; it really depends on the student. Detail oriented people can do very well on the SAT and struggle with the time constraints on the ACT for example. It's also important to remember that the scores are scaled, so your score is based on how you did compared to everyone else taking the test to a large degree. All California schools accept the SAT I and most require the essay if you decide to take the ACT.