The landfills are going to be full of polyester long after the natural cotton has returned to the earth. Our descendants are going to look at those polyester pants from a few years ago and wonder what we were thinking. In other words, use the poly if it's what you want. I like Guterman's. Yours is a better brand and should be fine. I used varigated rayon on my last quilt. I wasn't impressed with the thread's strength, but once it was in the quilt, I'm wasn't worried.