It would depend on the quilt top for me. If it was a masterpiece and expensive top I would go with Kona black for the back or see if I could find a wide back that would work. If it was a scrap top and it would see heavy use and washing, I would probably go with the poly/cotton blend broadcloth. I pieced children's quilts some 25 to 30 years ago in broadcloth and they are still going strong. My main rule of thumb is, I just don't mix cotton and polyester cotton.