I really depends on how the fibreglass piece is made, and how it's painted.
If the painting job sucks, fibreglass is going to have paint chips everywhere. Also, with any semi-hard hit the bumper will break, not flex. That said fibreglass is repairable too to a point.
Personally I'd have no problem using a fibreglass kit if I knew it was built well.
"Driving is like a narcotic, an addiction that can take over lives. It ruins people, breaks families apart, and even kills. Itís habit that needs to be fueled. Time, money, rationality, sweat, knuckle skin, and anyone who disapproves are of little importance to a driver, and are often overlooked. Hi, Iím Nelson, and Iím a drivaholic."