I have the M4 (the non cqb one) it's the non blowback version however, and it works great! *knock on wood* I have also handled the CQB version and givin the choice I would go with that one. The main reason is that the CQB comes with a crane stock (no unslightly battery options), a rail system, and considering its an armalite, your going to be spending mad money on it, so that saves you the trouble of buying a rail system anyways.
But it's your hard earned money, just pick what tickles you in that special airsoft spot.
Its an M4 so no matter what you do, you can always change it up!