Ive seen deer brought down by many .223 (the 5.56x45 is more potent, and a bit different in size, etc)at distances of 300 meters plus, I bet you would be hard pressed to get the 7.62x39 to hit anything aside from a barn at that distance. At that distance a 7.62x39 may not have enough force left to even do any damage.
The 2 cartridges do better at different distances. Why do you think the Russians got rid of the 7.62x39 and replaced it with a 5.45x39, to get the many more advantages of the smaller higher velocity round, which still does not perform as well as the 5.56x45 at longer distances.
If your goal is the cheapest wall of lead you can put up bar none, then get an AK-47. If you like the SKS for the way it looks, and because you want one, then get it.
There is no justifiable reason that the SKS would be better than an AR period, the 2 were made for 2 different purposes. The AR was made as a primary US combat rifle, a modern design, made to fix many long time problems inherant in combat rifles made before it. The SKS was a temporary fill-in until design, tooling, then production of the AK-47 was ready, and it could be distribuited.