I always think of muslin as being slightly coarser with less "processed" cotton in it, but I haven't actually shopped for it in many years. If you want to guarantee that the fabric will be softer, go for the non-muslin cotton. But I suspect that either muslin or other cotton would be fine.
Iceblossom is a kindly soul. I know this because she adopted me for a quilt show once
. "Life issues" got in the way of actually meeting up, but I'm still hoping we can meet someday.