We have not tried CTD or CTY, but have been doing EPGY for a little while. It was recommended to us by someone at Duke TiP the reason being that apparently Stanford provides some kind of 'transcript' that might be more likely to be accepted for credit by a school. Of course, if your school has told you just to take geometry, I guess this is not an issue for you.
We've been happy with EPGY so far (MOWD, honors pre-algebra); it varies the subject matter presented enough so DD doesn't get bored and then circles back around for some repetition--not too much repetition, so far. With EPGY it seems you pay a fee (~$530) for a 3-month period and if you finish the course early you can start a new one for a minimal ~$30 or so registration fee; if you don't finish, you pay the fee for the next 'quarter' and continue. I'm not sure how this compares to CTY fees but EPGY sounds more expensive if you take more than 3 months per course, which we are definitely going to do since we're doing about an hour and a half most weeks.
I'd be interested to hear more specifics about the logistics of courses and fees for CTD and CTY if anyone knows more; I found it a little hard to figure out from looking at the different web sites how they would actually compare.
