dmdeltesta said:
... a stainless cog or an aluminum one. ...
If you ride your bike hard, you can wear out a King aluminum cog in as little as a couple weeks. IMO, their use should be reserved for rich, gram-counting racers who are willing to spare no expense in their quest to win. In other words, for those who consider singlespeed cogs disposable items.
For the rest of us, steel is the material of choice. (When it comes to King, anyway. I've heard that Endless Bikes' aluminum Kick-Ass cogs are pretty durable. I haven't used one personally).
--Sparty