You can use either. The polyester thing is outdated back to when poly wasn't the quality it is today. I use cotton to piece, and I prefer poly to FMQ due to the fact it creates less lint and has less frequent breakage. I do like king tut as a cotton for quilting. I have found out recently though if you do machine embroidery on quilts to use a poly thread to piece because the cotton is weaker, and tends to cause holes in the piecing while embroidering so it was a tip from a class on machine embroidered quilts. Really it boils down to preference, as long as it is quality you should be fine