It depends on the look you want. A delicate satin might look better with a fine cord, something more rustic, bigger. It also depends how you wash the covers. You can get a slight shrinkage on the cotton (but you would probably only wash them on a low temperature). I have used both sorts and find the cotton easier to blend in at the joins but sometimes I could not get the choice in the shop. The cotton feels softer. If you are using a piping foot you are limited by the size of the groove. When I had lots to do, I sandwiched the cord inside a long length of bias fabric on the sewing machine, then ran it through the overlocker, putting the cord close against the foot. That gave me a length of easy to handle covered cord with a consistent width flange, so it was easy to put into the seams of the cushion (or cover).