Personally, I'd say G&P. It's a little cheaper and G&P's M4/M16 series are decent guns, had my M16 a couple of years or so now, the only thing that I've changed is the barrel to a MadBull 509x6.03mm, and it's still shooting the same FPS as when I got it. I've not opened the gearbox at all, it's not been cleaned, greased, checked or anything, and only today I've noticed that the trigger response has dropped quite a bit. I think it may be due to oxidising of the contacts, as I use semi-auto so often.
For the G&G, it's a Gen3 Top Tech, so comes with a MOSFET installed, which will eliminate any trigger contact oxidisation, so you won't lose trigger response over time due to that. Comparing my mate's G&G M16 with my G&P M16, the externals on mine are so, so much better. His G&G feels like it's cheap metal, whereas the G&P feels pretty good. The G&G feels kind of 'lumpy' when you shoot, it's hard to explain, but it doesn't feel as smooth as the G&P. The G&P has better trigger response initially and better fire rate.
He's recently bought himself a G&P M4 to replace his M16, because after comparing the two, he's said he'll never choose G&G over G&P again.
That said, most people absolutely love G&G and seem to recommend them against pretty much everything else. So, read reviews of each, check the dates on them too to make sure it's the same generation of each gun, and make a decision. Whichever one you choose, you'll have a decent gun that'll last a good few years on the stock internals, if you feel it needs it, put a tightbore barrel in it and you're all set to go.
