In a Red Dawn type scenario, I'd take the SKS. Simple, rugged, reliable, can go many thousands of rounds without cleaning, and needs very little maintenance. The soldiers in Iraq clean their M-16's and M-4's THREE times a day, that says it all right there. I'll take the SKS, or even better yet, an AK-47.
The Russians know how to build very tough, rugged, reliable weapons. They may not be 1 MOA accurate, but they will keep running under the harshest of conditions. The SKS is the chevy 3/4 ton, the AR is a Mercedes.
From what I have seen and read, in Vietnam, the M-16's jammed up at the worst possible times, during the first years of the war, during firefights = dead soldiers. Stoner blamed it on the ammo change and ball powder, once they finally chromed the chambers, it got better, but they still had problems.
A neighbor if mine is an ex-marine, he carried an AK-47 in Nam, no joke!