I have used the "super Muslin" sold at Walmart. I really like working with this because it holds its shape so well and is a nice color to offset pinks and browns (which I have been doing recently). But I notice that it is very difficult to hand sew but just the greatest for free motion quilting (no puckers). I will try what some of you call the "permanent press" or "quilters muslin". Sounds like a better weight to match the other cotton fabrics used in most quilts.
I had a nice surprise when I used it on my first "free motion" quilt. When I turned it over to the muslin backing, the pattern that was created over the whole quilt was so pretty! when I threw the finished quilt back into the washer and dryer, the finished product looked like an antique quilt with all the little puffs that emphasize the design.
Try it, you'll like it.