I think these days that either one is fine. In general I think that the ACT is curriculum-based (i.e. what you have learned) and the SAT is more geared towards general reasoning and problem-solving skills.
You could try and take practice tests of both and see which one you do better at. You could also base your choice on study opportunities. Maybe you know a group of other students who are going to get together and study the SAT. Or perhaps you know of a study course for the ACT that is half the price of the SAT. You might also consider the length. The SAT is longer so if you have trouble with the longer format, may want the ACT. The last consideration would be to look at the schools you want to attend. As far as I know, most will consider either equally, you might find that particular schools have preferences.