Take both the SAT and ACT practice tests from their respective prep books (I used the Princeton Review for SAT; ACT official red book). Then take both tests once officially. Find which test fit your style better and which one you scored higher on.
I have found the SAT is harder conceptually, but gives you more time to solve the problems, while the ACT is easier conceptually, but gives you less time to solve problems. Whichever test you took officially and found to be better for your testing style (and/or you did better on) focus on it. For me, I took the SAT (1140) and the ACT (19); I did so bad I just studied and retook both and got SAT (1200) and ACT (25). After my second time around I realized I was stronger on the ACT, so I buckled down took as many practice tests for the ACT as I could, and bought the Kaplan Math Tutoring online for basic and advanced math (just an hour long live tutoring session with other students) because Math was my weak point. I scored a 30 on my ACT after that and an 11 on writing.
Moral of the story, practice, practice, practice AND TAKE THE WRITING PORTION, don’t be that guy who scores super high but didn’t take the writing portion so West Point won’t be able to take your score (writing is also ridiculously easy on the ACT).