ORIGINAL: the_sandman_454
The biggest difference is the M16 is capable of burst or full automatic fire. The AR15 is the civilian (read: semi-automatic) version of the M16. To stay legal make sure you don't use any M16 parts that would convert it to automatic fire.
You might be able to find an M16, but it'd have to be produced prior to the late 80's machinegun ban unless I'm mistaken. It would also cost roughly the same or more as a typical small car.
Are you referring to the A1 or the A2? I don't know the characteristics of the A1 but I do know about the A2 and the A2 is not fully automatic. It's semi-auto or with the flip of a small switch becomes a 3 round burst (which is worthless). Also, I don't know about the AR 15 either but the M 16 holds up to 30 rounds and is a sweet little .22 that is capable of hitting a torso at 500 yards using nothing but a peep sight and well tightened sling tied to your arm like a turnakit.