If I'm not mistaken CTY uses Stanford's EPGY for math and Thinkwell/ Plato for science? It's been a while since I've checked. My guy (8.5yo) qualified for the CTY programs last year through SCAT. When I checked (a few months ago) it cost more than we could afford and the platforms used (EPGY, Thinkwell) were not particularly popular with the my son for math. We'd tried them independently when he was younger. We may use Thinkwell (through Thinkwell or the Homeschool Buyers Coop, not through CTY) for science at the AP levels though.
He started AOPS this year and loves, loves, loves it. It challenges him at just the right level I think. He's in one of the intro classes. I also like the flexibility of using either a class, just the books, or the books and Alcumus for added practice. Anything specific you'd like to know? I'll do my best to answer!
Good luck!
