Hmm, just a little bit of info. The stories are not related, only the settings are similar with may be a bit of parody here and there. Well, everyone thought it was a prequel which i can safely say its not. You don't need to watch C;H to understand S;G. They are only written by the same writer and company. Both titles are 5pb's breakthrough imaginary science ADV genre.
As for C;H, it was kinda a disaster for all VN players as the anime producers didn't even care to adapt it properly (as stated in their interviews). Might vary a lot for anime viewers though.