i love the wool batts but they are a bit more expensive- also DREAM POLY is wonderful to work with- and no shrinkage- lasts and lasts the dream company batts are all very high quality great batts to choose from. the poly blends are especially good to use in kids quilts/utility quilts they hold up to frequent washing/drying - make great 'drag around' quilts.
as for backing- one thing to consider when thinking of going with muslin- yes muslin softens with washes-
when quilting a solid backing will show every stitch- a printed backing will hide an errors in stitching. printed is much more forgiving- if using solid anything less than perfect is quite evident.