It's your choice. If you choose flannel, though, I recommend washing and drying *twice* because flannel can shrink an awful lot. Plus, I would recommend heavily starching the backing before layering, as washed flannel will have a lot more stretch than cotton (and therefore be more subject to tucks and puckers when machine quilting). Starch adds stability so the flannel will not distort during handline and will not fold over itself while you are sewing. My fast method for starching is to mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this solution on with a large wall painting brush until the flannel is saturated, wait a couple of minutes for the starch to penetrate the fibers, toss in dryer, then iron with steam.