Urethane is MUCH more durable, it will flex, and is not as prone to stone chips as fiberglass. I have a urethane bumper add on, and on the first day driving with it, I drove it over one of those concrete parking stops, the bumper folded in, but when I backed out, the bumper popped back out. If this were fiberglass, it would have been in pieces.
Fiberglass is a very rigid substance, especially in cold weather. If your driving your car in winter, I highly suggest you steer away from it, hits from ice and snow will crack it. Its much cheaper to produce than Urethane though.