I like cotton thread because I set my iron on Linen which is the hottest setting and sometimes I have to really press a seam open. Like as in, putting the iron on it, and letting it sit for several seconds. I am always afraid that I might melt the polyester thread if I do that. In my opinion, cotton does take a hotter iron without issues. Also, a lot of quilting I do is from kits. I don't always have any extra fabric to replace a piece that may get messed up.