[TCLP]

Knowing G&G its probably the same gun or one has a metal reciever and the other is nylon based plastic receiver.
That said what I do know is that an m4 and an m16 are mostly the same but have some big differences.
M16: has a longer inner barrel so it will have great range and fps based on the same spring. (ie if both an m4 and m16 have a m100 spring the m16 will have higher fps) Length is obviously longer which can be a bad thing if your a smaller person especially if you have a full stock.

M4: Shorter barrel good for moving around stuff, most come with telescopic stocks so it can be adjusted to your preference. Lower fps can be beneficial in some areas of the country which only allow lower fps.

Now with a G&G you will get a tightbore 6.04mm barrel instead of a 6.08mm, the gun will most likely have a ball bearing spring guide and is generally considered upgraded from the start. That said some people have had lots of issues with the mechbox in which they would crater fast. My has been good so far but it is relatively new still, aprox. 3000 rds.

JG will probably shoot to hot unless you live in Alberta as it will be about 380-400fps. Which means that you will have to downgrade the spring and if you don't know how that will cost you extra. That said some people would rather buy a JG before a G&G. Most JG that I have seen for sale also include an all black body where as the G&G do not.

Also please fill out your location, so more of the local guys can help point you in the right direction with fps rules and gun docs.

Hope this helps.