I am currently a college freshman and I took both the ACT (32, then 33) and the SAT (2290), so I can hopefully shed a little light. As others have mentioned, the science and math sections of the ACT can be a little tougher if you haven't seen the material yet. The other thing to consider is the difference in how the tests are taken - the ACT has each subtest in one, long chunk (i.e. 45 minutes for 75 questions in English), while the SAT offers parts of each subtest in 20-25 min increments. I personally "enjoyed" taking the SAT more because the smaller segments helped me pace myself with less stress and it was nice to alternate between English, math, and writing instead of working on the same subject for a long time.
