Will Canon's FF Mirrorless be EF or EF-M Mount?

First and foremost is the problem of perception if Canon were to go full frame EF-M mount.  Full frame EF-M instantly signals to every working pro that their entire EF lens inventory is now obsolete and will soon drop in value.  Think that might anger a few people?  If the long term prospect is that you will eventually replace every lens you own with a virtually identical EF-M version, what is to stop you from just starting over with Nikon or Sony?

Next up are the technical merits of the two mounts.  With the mirror gone, there are no optical designs that you could build for the EF-M mount that could not be duplicated in the EF mount.  In fact, the EF mount makes bright lenses easier due to the much larger throat diameter.

But what about mount adapters?  They suck.  If you are carrying multiple lenses with mixed mounts, the only option that makes sense is to put an adapter on every lense in your bag.  Your camera body might be thinner with an EF-M mount, but now a buch of your lenses just got bigger.  There are also several problems with the design of the current EF-M adapter; the internal baffle blocks a full frame image, it is not weather sealed, and it is not sturdy enough to support larger bodies and lenses.  There is also no physical blocking mechanism that would prevent use of the current adpater instead of a newer "full frame" adapter and there is no identification on the current adapter indicating that it is only for crop cameras.  That is a recipe for problems.

People need to stop thinking in terms of a full frame M2 + 22mm and start thinking in terms of a mirrorless 1DX + 600mm f4.