Nothing much now-a-days perhaps that the M2 has two o-rings on the tailcap end. The style of the M2 may be slightly different from the C2 but I've not seen a factory fresh C2 to be certain.
The C3 and M3 are very different. The M3 uses the "Millennium" bezel thread rather than the standard bezel thread. The C3 does not have an adjustable CombatGrip but does have a PocketClip (the M3 does not). The C3 has long flat sides compared to the round body of the M3.
The M3 body also has cooling fins...
Al