Like a few others, I quilt on a LA and first choice is always going to be a poly--it plays nicer in the machine, no lint, can use a thinner thread so that the texture is obvious without the thread overwhelming the piecing. I will use cotton if requested, but not a fan. For piecing I use both, as long as it's a fine thread. But I'll have to fess up--for stitching the binding to the back, I prefer C & C cotton wrapped poly--it doesn't twist and sews so smooth.