Because the SAT will change so dramatically, past experience with SAT prep classes are not comparable. (The old SAT could be gamed using tricks taught in those classes.)
S had signed up and even attended a few weeks of the SAT prep before he took the ACT. He prepped for the ACT himself using ACT books purchased from the local bookstore. Got the score needed on the ACT and dropped the SAT class. D used the same ACT prep books and got the same score. Both took the first ACT offered in their junior year. (By taking it so early, if needed, actual prep classes/tutors could have been used.)
The biggest issue with the ACT was developing the pace needed to complete the sections; timed prep tests did the trick.
I strongly agree that the earlier a kid begins prep, the better - that gives larger margins of error.
But because the tests are so important, don't scrimp on whatever prep your kid needs.