There are some differences between the ACT and the SAT, but I'm not sure that they're substantially different. For instance, the SAT penalizes students for guessing wrong. So if you get an answer wrong on the SAT, you lose a quarter point. If you lose an answer blank on the SAT, it doesn't affect your score one way or the other. On the ACT, for instance, there's no penalty for guessing wrong. What this means is that the SAT sometimes can be a little more coachable. That means if you can take the 4-5 answers on the SAT and winnow out the answers that are definitely wrong, there's a benefit to random guessing at that point. The ACT also has a science component on it, which sometimes kids who are strong at science feel a little bit more comfortable taking the ACT. However, much of the science component on the ACT is really a reading test. Can you look at data? Can you look at the question? And can you figure out from that what the right answer is? As opposed to necessarily knowing biology. It really isn't that. It's not testing you on scientific knowledge. It's testing your ability to manipulate data, to look at data, and come to some conclusion from it.