Track cogs seem to come in two basic types of steel....either stainless or some type of hardened chromoly.

Many of the most expensive cogs are stainless and when looking at two equivalent cogs, the stainless variety is more expensive. For example at Retrogression, the All City stainless cog is $34, while the regular chromoly cog is $22.

What is the difference? Do the stainless cogs last appreciably longer or is it just about appearances?

I have been riding fixed for a long time and of the cogs I have owned to date (Surly, EAI, Miche, Shimano, Soma, and a few others), all are made of sort of hardened chromoly, but never stainless steel.
I have gotten pretty high mileage out all of these cogs and whatever minor differences I noticed in drive train smoothness faded with the slightest amount of cog wear.

What am I missing? Is there some reason to pay more for a stainless cog?