C2 is a great light! I've run a HO-9 in my C2...baaaahhhright!!! All 6P/Z2/C2/C3/M2/G2/G3 run the same size LAs. Currently, I have an M2 head on my C2 with a Detonator extender.
The M2 bezel is shock isolated and thus larger. Not useful if you just plan to carry it. Any LA in a C2 head will put out more light than in a M2 head because of the LA is deeply recessed in the M2 head. My HO-9 puts out noticeably more total output, mainly in the spill beam as it's wider, in my C2 head than in my M2 head. If you just plan to carry the light, there is no advantage of the M2 over the C2. For performance, the C2 is a better option (more light, lighter, smaller). I just have the M2 head as I like the way it looks.