It's a tool, it's like asking which is better a hammer or a sledge hammer.

AK for close in work is brilliant, won't fail you in a pinch, cheap, has great penetration, blast through walls and doors at will, without alot of practice though only accurate out to about 100 yards, 200 if your good, after that your just hoping you hit something.

AR accurate, precision tool, lethal bullet do to it's unstable projectile once it enters flesh. Doesn't have the power or penetration of the AK, more accurate at long distance, (MUST KEEP IT CLEAN) This is a precision tool, although some versions are becoming more rugged, and more dirt proof.

M16 Full auto, great weapon, accurate at distance exact same as the AR just a little different platform. Like the guy said above if it's a full auto you need a license, but if you believe that the zombies are coming, and you feel comfortable hiding it, and not getting caught with it because you think government will (A) be your enemy in the near future so it doesn't matter, or (B) will no longer exist very soon because of their collapsing power base, and currency default then who really gives a shit, take what you can get and prepare right. I mean if we're going to be living in a world of no laws, I doubt if anyone is going to be asking you for your class 3A license.