I say G&P. I have used both and taken apart both.
If you're after external quality and realism then G&P wins hands down.
Real trades, engraved, much better quality and feel to the rifle than there is to the G&G one.
The G&P feels almost real whereas the G&G feels a bit toy-ish.
As for shooting, G&P also wins this one.
It has a much better rate of fire and trigger response than the G&G (both tested completely stock and with 8.4V 3700mAh batteries).
The G&P has a ROF of 22 RPS and FPS of ~340. The trigger response is amazing, good for shooting rapidly on semi, I couldn't cause it to lock up (pulling trigger too fast) if I tried.
The G&G has a ROF of 16-20 RPS and FPS of ~340. The trigger response is pretty good, haven't been able to lock it up either, but then I haven't specifically sat there trying to like I did the G&P.
The G&G has a problem on full auto - it doesn't feel properly and misses shots. Sometimes bad enough to get down to 10 RPS. This is caused by the sector gear and is apparently (according to Fire Support) a problem with a lot of G&G rifles.
Range and accuracy - pretty much even.
There's really not a lot of difference in the two. The G&P has a metal hop unit and the G&G has a plastic one, although they're both really good.
Gearbox.
They're both pretty good. Had them both apart, the G&P seems slightly better quality, and the sector gear and tappet plate are better in the fact that the tappet plate comes back further to prevent misfeed issues.
Overall, I'd recommend the G&P over the G&G.