If you're over 18yrs old...contact Shinobii and get setup with an ICS M4. Get an extra upper mechbox setup for 400fps and you're all set for both an indoor sub-350fps and outdoor sub-400 setup. It takes about 30sec to swap the upper mechbox.
The plastic receiver of the ICS is much better than the G&G. Thicker and more flexible. It'll give a bit...instead of the G&G plastic that is brittle and will crack under stress.
Also, aside from the plastic receiver, the ICS is made up of proper "traditional" ICS parts...unlike the G&G which is a complete "sportline" build, made up of downgraded parts from their normal line up.
The G&G blowback is nicely designed...but utterly useless. If any of those parts go...you're looking at trying to get specialized parts that aren't off the shelf. It's also weak enough (not in that it'll break...it's just kind of neat for the first mag and then just kind of laughable after that) to not really add anything of substance to the AEG.
Shinobii has or can get every part/piece/etc...of the ICS AEGs, so there's no issues with supply/availability (as long as customs aren't being too silly...but that goes for everything I suppose).