I have the WW kit on my CRX too. I also had to use Fuzor to acheive a perfect fit.
The differences between Fiberglass and Urethane:
Urethane is heavy and harder to work with than FG but it can really take a beating. FG is easier to work with and is a lot lighter than Urethane but much more fragile.

If I could do it all again I would get a urethane front bumper and the rest in FG. I have already hit a few curbs that would have destroyed FG.