fiberglass is famous for chipping.
urethane is famous for paint peeling off in patches.
if done right (professional-grade), you shouldn't have problems with either, although Fiberglass tends to chip more than Urethane tends to peel, if you get what i'm saying.
any rock, branch, etc that you hit with your Fiberglass front bumper is a chip waiting to happen. Fiberglass gives 0, while Urethane gives a LOT.