the majority of body kits are fiberglass. if style is what is desired you'll mostlikely have to compromise and yield to the material. if you want to go fast, then the lighter fiberglass is your ticket.
i lean towards urethane, because my car is lowered. and i'll be damned if i crack the front end if i misjudge a bump. spider marks will mostlikely be the result of what i get with my urethane if i made an error. just with urethane selection, nothing is aggressive, all are fairly conservative.