...I don't know, I am just a "chairsofter" guy, but I got a JG - MP5 and I think that its awesome.
The upper and lower are both metal, as is the adjustable shoulder rest.
The thing is virtualy all metal, and it has always been really reliable for me.
Mine has the black gear box in it, which has 7mm bushings in it and I have never had any issues with it.
For the money, I really like Jing Gong.
...Having said that though, I don't game my MP5, so you are probably better off listening to these other guys who really know what they are talking about.
Later
Gordo