fiberglass is lighter but more prone to crack but easy to repair. urethane is more durable, yet more expensive, and sags over time. i had a high quality fiberglass kit once, let me tell u, it is hard to keep it perfect, cuz u cant. rock chips crack it, slight hits cracks and what not. urethane is kewl but is also heavy and very hard to paint if the shop doesnt put a special flexing agent into the paint.

both sides weight each other out. with the money u spend on the kit and paintin, you could be doin better things with ur car else where. go look in the appearence forum to get a few ideas before deciding to pick a kit up.