Right now the SAT and ACT are paper-and-pencil tests, that's why you want your student to get paper-and-pencil prep and not take a class where they're answering questions on a computer screen if that's not how their test is going to be administered.
You also want to make sure that any class your student takes includes both the content they're going to be be tested on, but also the strategies they'll need to succeed. You don't just want a teacher who's good at math or good at English but doesn't understand how these tests work.
Finally, you want a program that's going to help students build over time. There are a lot of programs out there that just have students answer a lot of questions or take a lot of practice tests, assuming they'll figure it out eventually. Don't think of SAT prep or ACT prep like track practice, where if you run more, you'll get faster. Think of it instead like building a house, where you build a solid foundation, then a really good first floor, before you build the second floor and put a roof on it.