Think of how you're going to baste your quilt. I usually spray baste and have always used cotton but tried poly on a small quilt. I discovered that the spray didn't stick to 100% poly batting. The quilt sandwich looked fine when I sprayed it but, as the quilting progressed, the spray disappeared and I ended up pinning the rest. I have used Joann's Warm & Natural(?) 80/20 blend without problems, the 80% cotton keeps the spray active. I mentioned this to my LQS instructor and she also said that spray basting doesn't work well with poly. Now, if you use a long-arm you won't have this problem and I like the poly because the extra loft will better define your quilting design.