No discussion on ICS M4? Just need to shave the 2nd tooth down, put a new motor, and it's a great piece IMO, especially with the rotary hop up design.
The writeup on G&P M4 is far too charitable. Their mag-well is warped. That has an effect on the mags that you use and *some* air seal issue. E.g., on my G&P M4, even after the velcro mod, leaning a MAG 130 rnd mid-cap forward gives you an extra 10 FPS. You lost about 60-80 FPS on a Beta Project PMAG mid-cap.