M4A1
Pros: Low recoil for easy bursting and control, fully customizable, only 800gp (unless defaulted)
Cons: Weak 4-6 hit knock out
AUG A1
Pros: 6x Zoom, 3-5 hit knock out, 2 firing modes
Cons: Hard to control, 6x Zoom
The AUGs 6x zoom can sometimes be a great advantage, but in some situations it zooms in too far and its hard to tell what your aiming at.
The M4s weak power makes it a bit difficult to down heavily armored targets such as a Heavy Vest or Tanker Vests, but its low recoil and good accuracy insures each round hits its target.
Subject: AUG A1 or M4A1? 
