Just by looking at the x2 frames you can see differences between MC1 and MC2 =
In a nutshell, handlebar/stem are totally different including the way they in the steerer, MC1 has a quill, MC2 is a collet.
MC1 chainstays are square, MC2 more rounded.
MC2 frames are a lot lighter and the tubes thicker in diameter.
Exit/entry holes for the cable routing are different.
MC1 has horizontal dropouts, MC2 micro dropouts, I think they call them.
MC1 frames have the more "classic" colour schemes, dolomite/backfire/more linear fades, this makes them abit more popular than the MC2's.
There are other subtle differences.
A good site to look is www.oldklein.com
Hope all this helps.