I'd much rather have an M4 instead of a CAR15. its newer, and can do anything the M16 can do.
However, my true favorite is just about to be introduced. Anyone else seen the specs of the new M8? Seen the thing burried in mud, picked up, and fired accuratly at a target over 100 yards with iron sights. THree variants, Assult, Sniper, and support. If i knew how to do the images here, i'd post one.