First off, are you going to install it yourself, or have a quality shop do it?
Urethane will be the easiest to install, because it will stretch to fit, as Corvettes are built on a jig, and no two vette bodies are identicle.
Fiberglass/Flex Fiberglass will hold up much longer than Urethane, barring any accidents. I have a fiberglass rear cover and am installing a flex type fiberglass front bumper out front currently. I would recommend the flex type glass, it can be repaired like fiberglass, and is a little easier to fit than regular fiberglass.
On the Urethane side....
I have owned a dozen 78-86 Camaros & T/A's, and have never had any trouble with the Urethane bumpers on these cars, and all of them were outside year round . On my vette, the bumper covers were so hard and cracked, I literaly took the bumper covers off in pieces, then removed the retainers. My vette has been painted a couple of times by P/O, but since "rotted" O/E covers is very common, I don't know if it has to do with the use of laquer paint at the St Louis plant or the quality of the factory Urathane from 73-77. I do know if the propper paint/primer isn't used, it can fry the Urethane.
