for quilting I use cotton but, for embroidery I normally use rayon for the shine. That being said, all the professionals will tell you and I agree if you are going to wash it alot then go with poly because it holds up better. I use cotton on my quilts because its suppose to last over time better, cotton with cotton, where poly would eventually start cutting thru the fabric. I don't know if that's true but I went with it. I've heard lots of people using poly but I'd go with what I felt comfortable with. Maybe switch with the next quilt and use this one as a test to see how it holds up