The ACT has four sections if you don't do the optional writing section: math, reading, english, and science. Math and reading should cover similar items but probably not in the exact same way. English on the ACT has some overlap w/ the multiple choice part of the writing on the SAT.
The SAT has a history from years past of being more of an aptitude test not straight achievement, but it really was reworked a while back to be more achievement oriented like the ACT. In other words, they are both now designed to test what a high schooler has learned through things he has been formally taught. The verbal analogies we all did on the SAT, for instance, are no longer there. Those types of questions got more at ability and vocabulary than do the types of questions they ask now.
My 8th grader took the ACT this year so she'd have a chance to try both the SAT (last year in 7th) and the ACT this year before they stay on her permanent record. ACT did tell me that they don't wipe the scores out at the end of the year like SAT does for middle schoolers, but that they never send a full report with all of your ACT scores from all testings when you have them sent to colleges. They only send the one test date you request.
