Here's an excellent and simple comparison of these two standard college-entrance tests:
SAT Vs. ACT: Understanding The Differences. It seems like a must-read for every high school sophomore or junior! For example, if you lean toward the sciences or English is your second language this article will lean you towards the ACT. If you are more into the humanities or the legal profession then the article leans you towards the SAT. So far as I can see, most colleges accept either, but you'll also want to be sure to check the admissions requirements for your favorite schools!