Im looking into both of these guns for field play and i like them both but the m4 is more compact and easy to manuver but the m16 has a longer barrel meaning more range and accuracy. i plan to use a m120 spring and i want a gun that hits hard, works well, has good accuracy, and stays around 400-415 fps with .20. I do plan to do some internal work such as better gears, a bravo 1 mosfet, shims, etc. and a lipo. i really would like to hear from someone who has experiance with one or knows about them.