Over fifty years ago, almost all sash cords were vegetable fibre that could be attacked by molds or simply become stiff and weak from age. It used to be quite startling to have a fifty year old sash cord break during the night, and cause the sash weight to fall with a thump that would wake up all the occupants of the house. Nylon cord would undoubtedly last much longer. I have no idea how expensive chain would be.
But the newer devices take up less space, are easier and less expensive to install, are less susceptible to heat loss and drafts, and work more smoothly.
I found several sites by entering "pictures window sashes cords sash weights" into Yahoo! search box. But none were good enough to tempt me to post URL.