I had the stalker for awhile, it was urethane. You could twist it all around without it cracking or tearing. I hit something with it though, and it tore right where it screws into the fender on both sides and the thing ended up face down the road. I'll buy another one eventually.
That bumper used to take alot of abuse since it's lower than the stock and sticks out further on the bottom. I was coming out of IHOP one day and the parking lot had a mean angle to the road, the bumper hit the road damm hard, with enough force to snap both of the bolts on the retaining bars that held the bumber out and stable. It I just put new bolts in and the thing was back in shape and looking good.
ANyway, I would stay away from fiberglass generally, but the point about fiberglass being easily repaired is a good one. Still, it would be hard to match some heavy curvature if it were to rupture.