Our experience: EPGY is fun, fairly easy, and quirky. It has mental math, which is great. Coverage of subject areas is unpredictable but broad. There is NO option for practice or further depth: the software is supposed to give extra practice if you're having trouble, but I'm not sure about that. Word problems are well taught but not extensive. The coverage of logic and stats is welcome.
The EPGY open enrollment content is, as far as I can tell, exactly the same as the full-pay course. We've done both. With the full-pay course, you get access to a tutor, whom we never contacted although we could have. You also get a grade but only if you complete the class and take a final exam.
At CTY online, DS took a problem-solving class. We liked it very much except for the disjunction between the course content and the testing. The course content is something called Destination Math. Very logical, slightly geeky cartoon presentation, but clear and offers good options for more depth. The course content wasn't really very hard, but it did teach DS something. The exams were WAY harder than the course content and often required big leaps beyond what was taught. Some of the exams took 2 hours to complete, and this is a kid who tested in the 99th percentile for math. The exams at points were more like AMC-8 problems (only harder) than like a test of the material taught. We asked the teacher, who just said, hey, it's for gifted kids; it's supposed to be hard.