It's probably me and ONLY me but am I the only one that can’t run in polyester? Every time I attempt it I overheat. The sweat gets trapped under my shirt with nowhere to go. Cotton on the other hand absorbs sweat but in doing so seems to cool me down. I know - 1 vote now for cotton and millions for the other. Then there's that chaffing in the nipple area. I promise you with cotton it'll never happen.