Poly is a LOT less expensive and it holds up wash after wash just fine. It's warm, it's comfy, and it's a little fluffier. Sometimes I like it better than cotton. If you have problems with shifting (I never have), just use a lot more pins or spray basting if you prefer. I think that if it didn't work well, there wouldn't be so many types available on the shelves and manufacturers wouldn't keep making it.