text: The 'poly' you are referring to is polyvinyl chloride. It has a number of advantages over iron.\n1. Iron corrodes (rusts) and requires painting every few years to protect it.\n2.  Iron is heavy and so heavier stronger brackets and support is needed and will be physically more difficult to put up.\n3.  Polyvinyl chlride is much easier to cut to any required length.\n3.  Polyvinyl chloride is cheaper than cast iron