I'm not aware of a difference in materials of the factory originals between 79-81. In my experience with my project cars, they all deteriorated over time becoming brittle and broken, unless the car was a low mileage, garage kept car.
The reproductions I have are very nice, and probably made of better stuff than the originals. Seems to be more flexible. I bought the ones that are made in Australia, and am happy with the quality. Here they are:
http://www.ebay.com/itm/79-Pontiac-F...-/360915687918