I vote wool, too--it breaths well so always the right temp despite weather, it is a dream to work with, it has nice loft but without weight, and don't worry about the bat showing through the white--I used Hobbs wool on a modern quilt like this with 1/2 of it being a white negative space area--was a little worried that the natural color of the wool would show through--no problem at all with that.
Lovely quilt!