There are several posts on this already that go into greater detail. Just do a search and enter this forum as the forum to search in. Type in "MXC EXC" and every post that has those two words will come up.
The short version is this: The MXC has the close-ratio tranny, the EXC has the wide-ratio. The MXC has the lighting coil, but no lights. The EXC has the lights already installed. I believe the suspension is the same. I'm not sure, but the flywheel weight may be a bit lighter on the MXC (I's have to verify that one). And there may be other differences between the 4-strokes. The ones I listed are two stroke specs, but I believe they hold true for the 4's as well.