W&N definitely makes a quilt feel heavier then poly but actually poly is warmer because it doesn't breath like cotton so will trap body heat better. I like the drape of W&N. One thing to consider, though, are you handquilting or machine? I find W&N difficult to needle when handquilting. It is a dream to machine quilt. Poly is much easier to needle for HQ, next would be the 80/20 blend. 80/20 may be the best of both worlds for her because it will have the weight of cotton and the insulating powers of poly. So I would go with the 80/20.
I do not prewash batting because I like the crinkled wrinkled look it gives after washing. Prewashing may also affect how it hand quilts. It is a nonissue for machine quilting IMHO.