Here we go with the old "ar vs.ak" shit.
An sks can be quite accurate. Check out my and Bryans prairie dog pics in the hunting section.
Anyways, environment should choose your weapon.
Those who think they will be in the city and taking 600 yard shots are few-realistically, an sks would be better than an ar all the way around. Accuracy is acceptable to 100 yards and most likely you would never be shooting more than that in an urban or even mountaion enviorment. If you were out in the flatlands a .308 of course.
An sks has some distinct advantages at 100 yards, namely power. Especially if you were to be shooting at cover. And an sks will shoot anyone center mass just as well as any gun. I would prefer the power of the 7.62 anyday.