I have an E-Series M4, and the only issue I had regarding the gearbox was a stripped piston Otherwise, it's a very nice gearbox. G&P gearboxes are rock solid, so if the M16 comes with the same gear box they sell separately, I don't believe you'll have any issues with it. Plus, it comes with a G&P 120 motor if I am correct, which is also nice. So I believe you can't go wrong with either gun if your concern is merely the gearbox as they are both high quality in my experience. However, the externals are a bit different, but both high quality. G&P's never close the dust cover, and VFC's not only close the dust cover, but can lock back the mock bolt so that you can adjust the hop up easily and then release the mock bolt by pressing the bolt catch as well as a slimmer pistol grip. It's pretty much preference at this point. I will point out that my VFC is VERY finicky with magazines though.