personally, I like the ICS product. It's like the aftermarket PTW. The best thing about ICS product is the easy swap cylinder for the indoor and outdoor game. It won't give me headache to swap the spring and put it back together on the ICS M4, just need to take one pin out and swap another cylinder. That's all you need to do on ICS M4.
G&G product is quite pretty good build but still they are using V2 mechbox. One of the mechbox that I had bad experience with. It will give you a hard time to swap the spring to go on the fps limit according to the game unless you are an AEG expert.
I like all ICS Product that I have from M4 to L85A2 ( my favorite ). so go for the ICS product, you can't be wrong.