G&p every time, they are much better, g&g have a little blowback which tbh isn't that great.
A g&p m4 has pretty much the most solid externals of any m4 without going into systema and inokatsu prices.
If you want to upgrade it, a new hop rubber, maybe a tightbore, that's all you need.
G&P is the best m4 under £400