Both are fantastic examples of what very good genre TV can be when you really try. Hush came at a time when the show was still near the top of its game while Once More, With Feeling was released when the show was on the decline (but still good). I think it made OMWF stand out a bit more but then again, I've never gone back and rewatched an episode of TV more than OMWF.
Once More, With Feeling takes it.