It used to be that here in the South, the SAT was exclusively accepted, but for the past couple of years, my students have been able to choose between the two or take both. Many of mine choose to take both and then determine which one fits their test-taking personalities better. Here are a couple of factors to consider when choosing between the tests.
1. Do you do better on a test if you are able to focus on one subject for an extended period of time? If so, then the ACT is problaby for you because you take one subtest for each subject and then you're done with that subject. In contrast, the SAT presents you with shorter sections of subjects and jumps around a lot more than the ACT. Many of my students like having the exam so broken up.
2. Do you want to have the option of skipping questions entirely and not being penalized for that? Then the SAT is for you. You won't lose points for not answering questions, but you do lose points for answering incorrectly. For the ACT, it is better to answer every question because your score is based solely on how many you answer correctly. You are not penalized for incorrect answers; so you don't want to skip questions.
3. Do you want to have the option of whether or not you take the writing section? If so, then take the ACT; the writing section is optional. Several years ago, the SAT added the writing section, and it is mandatory, but what is frustrating about the writing section is that it lengthened the exam, and most univerities do not even consider your writing score when looking at your SAT score.
4. Are you taking Advanced Placement classes, especially an AP English class? If so, then you might want to take the SAT because it is written by the same company as the AP exams (The College Board). While the scoring is different for the exams, if you get accustomed to the AP English exams, you will probably think that the SAT is an easier version--this could help with test anxiety and increase your test-taking confidence.
5. Finally--and this is not a proven point--many of my students who struggle in math have told me that they think that the math section of the ACT is slightly easier than the SAT's math sections.
In regards to practice books, I have found the Kaplan books to be the most helpful to my students, but The College Board does have a blue review book that is also quite helpful.