Binkley, if you look around you can find threads that discuss the various merits of wool vs. fleece (by fleece, i understand polyester fleece, not something of cotton, which is pretty much junk). I don't think there is any practically significant difference in warmth between the two materials. I have been served equally well by both.

What is critical is the thickness of the material. Polyester synthetics aren't eaten by moths, but wool is more fire resistant. Synthetics probably dry a bit faster and don't hold as much water. Either one will keep you alive to see another sunrise....