Students along the East and West Coasts generally take the SAT, and those in the middle of the U.S. generally take the ACT. I took both, and I must say I thought the ACT was easier for me. I had to work a lot harder on the SAT to boost my score up. In the end, my SAT score and my ACT score were pretty close (2270 vs. 35), but my ACT was still higher (a 2340 is a 35 ACT equivalent). For some people, it's the exact opposite. It depends on how you think.
The SAT involves more reasoning and less of your absolute high school knowledge. It's a longer test, and you don't feel too terribly rushed on it. The ACT involves less reasoning and more of what you've learned in high school. It tends to go by more quickly, and students generally feel really pressed for time (especially on reading and science). It just depends on how your mind works (some are equally good at both; most people are better at one than the other).