Never used the G9, but from a comfort level I really enjoy the G5. If have to replace it with something else I would choose a similar shaped mouse instead of the G9 which looks uncomfortable to me. Someone which have used the G9 will probably come and prove me wrong, but I'll stick to the shape I know to be comfortable.
Don't know too much about the G9 though so can't really tell you what the difference is, apart from the obvious higher dpi on the G9.