Polar fleece is warm (but not as warm as wool just from experience, I own polar fleece jackets that I do not think are warm at all) but they definitely have a wicking property where they will wick the sweat right off the horse, and move it to the outside of the cooler, you can actually see it if your horse is very wet. It's awesome!
My new favorite is my Schneider's chameleon cooler. It goes all the way up the neck, but is fitted (not a square, which I like, but not practical for the trailer or for their stall), it does not get static cling like fleece, and it wicks away the sweat very well, and it's warm. It is similar to the Dover's chillchasers, which I love (both the look and the material) but the neck holes in the Dover's one are so big that my horses have no chest coverage in them (they hang down below their chests). They would work well for a large bodied horse, or possibly if you could alter it or order down a couple sizes.