AEG differences:
M4 cqbr - for noobs, first (stock) gun. No upgrades. Rattling hicaps.
Mk18 mod 0 - ultra reliable semi custom that keeps on going. First try at paint jobs. Triangle sights ftw.
Mk18 mod 1 - upgraded to the hilt blaster for gear whores, seal wannabes, guys with too much money, fancy RS optics, nvgs and no kids. Sometimes has never seen a game, either hanging on the wall or always on the workbench.
RS differences: See
http://en.wikipedia.org/wiki/SOPMOD