I think when we talk about serger thread, or cotton thread, or quilting thread, or polyester thread or any other kind of thread, it is good to mention the brand, the weight, what it is made of as "polyester" tread has a lot of variables. For example, I have found that King Tut, mettler, gutterman, sulky, coats and clark, wonderfil, YLI, Superior threads, ect. All make cotton thread and there is a lot of variety between them. As well as the thickness. So my answer, when folks ask is cotton or polyester thread better for piecing, I would ask back what brand and weight.