Rev, I thought you wanted an M3??
Inertia (M1,2,3)-
-Simple, Easy to clean, less parts to break, lightweight.
-Gun needs to move rearward under recoil so any extra weight or a tight grip and the gun may fail to cycle with lighter loads.
-Recoil more than gas guns.
Gas (M4)
-More complex, hard to clean, heavier than inertia guns.
-Gas operation means the gun will still cycle even if you bolt a tripod to it.
-Extremely low recoil
Now if someone said I had to decide between my M3 and M4??................. I would uppercut their scrotum, take them both, and run away.