Do I understand that the M16A1 has been cut to 16", but maintains the original gas system in the front sight base?
There are some compromises with that arrangement; since there isn't much dwell time in the shortened barrel, past the gas port. The M4 will have the mid length gas system, which was specifically created to solve those problems.
I have the Brownells 16" carbine, with the original gas system; reputedly the gas port has been enlarged to compensate. My carbine has been cranky with anything but a Colt BCG.
Anyway, if serious use is your plan, go with the M4.
Moon