urethane is going to flex more and be more durable when hitting bumps and other things if your car is lowered, but really, if you have a body kit you shouldn't be hitting stuff b/c you should drive careful, but sh!t happens. the fiberglass is going to be a little cheaper but if you hit anything, it will have a better chance of cracking instead of flexing like the urethane would. i've heard something about urethane being a lil harder to paint, but i dont know about that, maybe a bunch of BS. a urethane kit would be your best choice, but there isn't as much of a variety as the fiberglass kits and the urethane is going to be a lil more expensive.