Urethane is way better, it is flexable, basically the same material as your bumpers, I have a fiberglass kit on my honda and it breaks and cracks all the time, I wouldent even consider buying fiberglass, go with Urethane hands down its pretty much unbreakable unless you crash, it would pretty much break like your bumpers would, it would take a pretty big impact, your car would be FUBAR anyway
I have a fiberglass kit on my car. I hit a couple of parking curbs when I first had it put on. I have not cracked it or caused any serious damage. Urethane is definately more durable and forgiving. Fiberglass is much lighter though.