All the aeg's on BuyAirsoft have a nylon upper and a plastic lower. The metal parts are everything but the pistol grip, sliding stock (the tube is metal), foregrip, upper and lower receiver, and I believe also the carry handle. It's not full metal, nor are any CANsoft guns. G&G's from higher end companies (such as Mach1Airsoft) have upper receivers that are metal, however only when stated. In terms of which one you should get, it depends where you will be playing the most. If it's outdoors in wooded areas, you may want to lean towards the M16 more as the increased barrel length can help accuracy in some cases. However, if playing in a more urban environment go more for the M4 CQB or M4 Carbine. If doing a bit of both (CQB and larger areas such as woods) go for a M4 Carbine, as it's a mix of both decent barrel length, but still compact enough (especially since the stock can extend or retract).