I personally use Cotton on my quilts. I use all cotton fabric, so I can't see using synthetic when using all natural products.
Yes the loft of a poly is great to see the quilting, but can be extremely hot (especially for women my age).
My new batting I'm using is Legacy by Pellon, it is an 80/20 like Hobbs, but a tad thicker.
I have Warm/Natural, Black Hobbs, Hobbs 80/20 in both queen and King and Legacy's washable wool.
I'm a batting junkie.
Don't ask about thread, that is a whole nother topic. :)