John,
If you have not had the opportunity to do so, you may enjoy the data in comparing Steel Vs. Brass in an AR-15 in our LuckyGunner Labs section. A direct link:
http://www.luckygunner.com/labs/brass-vs-steel-cased-ammo/
I would suspect the findings may be similar with the AK or SKS, depending on the manufacturer of the rifle. However the chambers are designed more towards steel cased rounds with looser tolerances. I noticed a significant difference when trying to resize brass 7.62x39 fired through an old SKS vs. an AK-47 with a new aftermarket barrel. As you may guess, the SKS was more difficult to resize.
Some will argue that its worth firing the steel rounds over brass in terms of saving money, and with a higher quantity of rounds fired, the savings could buy a new barrel. Over time, I usually fire a mix of steel and brass for a number of reasons. With the above concerns in mind, this would certainly prolong barrel and chamber life. Those who fire a bolt action rifle in 7.62x39 will demand brass only for their rifles.
I hope this information helps. Please let us know if you have any additional questions!
Eric