I have only used polar fleece on me. My girl has a wool cooler which is really nice when she works up a sweat in the cold and has to trailer home in a stock type trailer. She does have a winter coat but it isn't much as she is a TB. I really like the wool. I think it is more "solid" than polar fleece and therefore probably warmer. I haven't yet washed it but I imagine it will be fine if washed in cold water with mild detergent and line dried.
Hmmm, ok. So if wool is more breathable, I wonder why I don't see more of it out here where winter is pretty mild? (Not gonna lie, it was nice to go out today and ride in the 65 degree sunshine in a short-sleeved polo!) My mare rides so much better with her water-proof quarter sheet on when the temp does drop below 55, I was going to get her a fleece one. I'm now considering wool. Especially if I can wash it. Hmmm.
Any more thoughts?
In California, we don't so much have a tradition of those warmer items for a functional use... mostly either waterproof or fashion. People moved away from wool for the advantage of washability and price... and are moving back to it again.
Wool is still warm when it is wet and it has a lot of advantages. It is much more breathable and insulating than polarfleece, so more acceptable over a range of temperatures. An advantage that probably hasn't occurred to you but is very practical from a California viewpoint ... is that wool is highly fire resistant and that fleece or any synthetic is deadly in a fire. So if for some reason you found yourself fleeing a fire, a wool blanket would come in real handy.
If you are allergic to a thing, it is best not to put that thing in your mouth, particularly if the thing is cats. - Lemony Snicket
While I love the look of wool, after looking into the pluses and minuses of both, fleece is supposed to have much better wicking quality than wool, bringing the moisture away from the horse's body, unlike wool which lets it sit there.
That's not been my experience. My horses with a wool cooler on dry about twice as fast as a horse wearing a polar fleece cooler.
I think one of the issues (besides the convenience of "wash and throw in the dryer") is that a good quality wool cooler is EXPENSIVE. For everyday use, polar fleece is probably the choice of many horse owners.
I agree with another poster...Irish knit under any cooler if you want the horse to dry quicker. You trap a layer of warm air between the Irish knit and the cooler which allows the moisture to wick away through whatever material you are using.