When choosing your test prep, many students prep for the SAT and then also take the ACT, because most students find the SAT trickier. When you are at the test prep place, they will help you decide which test to focus on.
All colleges accept either test, so there is no worry about which test to take. Most kids take both tests once, see which test they do better on, and then take that test for the second time. The second scores are usually the best scores and are usually the ones we send to the colleges. Again, all colleges accept either test, and we only need to send one. The colleges are not allowed to prefer one test over the other. We can send only our best score and we do not need to report all scores. This is called Score Choice.
Some schools, like the University of Washington and Pomona, requires that students send all scores from one test. So if we send the SAT we must send all scores from the SAT we have ever taken. The same applies to these schools if we send the ACT. No college requires that we send all SAT scores AND all ACT scores. But some colleges do require that whichever test you decide to send, you have to send all of the scores from that test.
I typically suggest that students prep for the SAT, take both tests once, choose the preferred test, then prep specifically for their weak spots on that test, and then go take that test for the second time.
I also suggest that you be doing the test prep in the summer before Junior year or certainly in the fall of Junior year, to start testing in the fall, or January at the latest.
Remember, you can post any questions about the SAT and ACT in the CollegeMapper Forum! Good luck, everyone!