also if you want to invert your gimbal and put it on an Alta only the M15 and M5 can do it safely. I'm still not sure why freefly have not developed an upgrade which would allow you to do this. Seems a little inconsiderate to early adopters.
The M10 is a bit of a tweener, and its biggest issue is not weight capacity, but having enough space to balance a longer, front-heavy rig. Unless you are very sure you only ever want to fly a DSLR sized camera (up to a C300), I would strongly advise getting the M15. For example the new Red Weapon cameras are fantastically light, but once you chuck on a lens + filter + mattebox (I'm talking the lightest kind here, like a Bright Tangerine) + remote follow focus, the rig's less-than-ideal center of gravity will mean that, when balanced, the back of the camera will hit the tilt bar when mounted on an M10. This can be circumvented somewhat by adding counterweights, but that is not ideal when the goal is to make the rig as lightweight as possible. The pan bar thing is also another big deal (being able to safely invert the gimbal). You'll probably want that kind of versatility.
I recently had an Epic-X with varying Canon lenses on the M10. There was no Matte box but were using a Preston single channel remote focus. I never once had an issue with the CG being too far back.