Maybe a better example would be helpful. Suppose you had a Nikon F4 (which I still own), and wanted information on how to use one of it's features. Which of the following would you rather use:
a) a forum for the Nikon F4, or
b) a forum for all Nikon F cameras
For those who simply come here to read, and enjoy the discussions, one big forum is probably better. For specific answers, despite the similarity of the cameras, two separate forums are better.
There is a third option - create three forums. One could be for M-series cameras, one for the M8, one for the M9, (and later on, one for the M10....). That way no decision is necessary, and people can vote with their fingers. If I were the forum organizer, that's what I would do, and leave it up to the users where they want to post. In a few months, it would all sort itself out. 🙂