Hi, Ania,
DS7 is currently taking both. CTY Elementary Problem Sovling I in math - clearly way below the level you're looking at, and EPGY LAW for grades 3-6.
I've done a lot of online reading, and what I've read is that for the lower level math, CTY uses the EPGY software, so there is no difference whatsoever, except for the fact that EPGY is cheaper. The Elementary Problem Solving Courses are exclusive to CTY and fun for young children (much more vibrant with animation, funny characters and content). But I read that the CTY algebra and above unique software is better (or maybe more interesting?) than EPGY. I'm sure you can search and find the same reviews.
I also heard from the psychologist who tested DS that kids love the CTY science courses, but not the language arts courses. But that may be the in person summer programs - not sure.
DS is taking EPGY language arts, and it covers all the bases for grammar, but it's rather dry, boring and repetitive. If you go to the courses page, you can click on any of the courses and see what's covered and try the demos. Also, DS objects that if he gets something wrong they rerun the lecture and rerun the lecture and rerun the lecture. Just what the research says not to do with GT kids. But that's easy to solve - DS is allowed to run around while a lecture is playing (yet) again.
One thing he does (sort of) enjoy is watching his progress after every session. So far, the course is very easy for him, but I can't convince him to do it very often. I believe when you get to the sixth grade level, there is writing involved, too. And there are a lot of exercises. No matter how good you are at the course, it takes a long time to get through them all. DS has done about 500 exercises so far with over 90% accuracy and yet he's only at grade level 3.8 or so (it started at 3.0 in January).
I know nothing about the 7th grade and above programs, so none of this is helpful to you (but may interest others considering these programs for younger children).
Finally, the EPGY tutor is more involved and checks in weekly and emails encouragement. She also holds whiteboard virtual classes. The CTY tutor has emailed after every quiz or test, usually with the wrong info (e.g., referring to a problem on someone else's test), but that's just luck of the draw I'd guess.
Sorry I can't be more helpful.
