Testing has shown that the difference in insulative performance of synthetic and wool fabric of similar thickness and weight is negligible with a slight edge, if any, going to synthetic. Base layers do two things, control temperature or insulate and control moisture and the later is where the differences in wool and synthetic become obvious.
Synthetics are lighter, dry faster, less expensive, do not absorb water and most modern versions handle odor pretty well.
Wool won't melt, is naturally resistant to stinky bacterial growth, and is a renewable resource but it absorbs water (but not much more), dries relatively slower (a lot slower), is relatively heavier, doesn't wick as well and is more expensive. Many users say that wool feels less clammy and warmer when when wet or damp than synthetic.
In my opinion, the current crop of top of the line base layers are so good, the choice between wool and synthetic really boils down to personal preferance.
Wade
www.HardcoreOutdoor.com