A plug for horsehide if you want thin
When you compare horsehide holsters to those made of cowhide, you see that horsehide tends to be harder, stiffer and more dense than cowhide. I'm sure that horsehide is difficult to work with because of these properties, but the net result is that a holster made of thinner horsehide will have the same strength and durability as a holster of the same design made of thicker cowhide.
So if your objective is a thin, less obtrusive holster for IWB use, you should really look at horsehide. The Milt Sparks Company does a lot of work in horsehide, and they advertise that their Watch Six horsehide IWB design may be the thinnest IWB holster on the market. I think they may be right in this claim. There are other makers who use horsehide as well - not just Sparks.
On the downside, horsehide doesn't seem to "give" as much as cowhide, so initially the holster may not seem to conform to your body as well. But I have found that horsehide will eventually conform to you, if you simply wear it and be patient. It is a slower process than with cowhide.
2% price premium for horsehide
If you go to the Milt Sparks website:
Originally Posted by QKShooter
and click on the page describing the Versamax 2 holster, you will see that the cowhide version (VM-2) is priced at $100. The horsehide version (VM-2H) is priced at $102. That is a 2% price difference, which seems quite small.
Perhaps Sparks has discovered a low cost source of dead horses?