fiberglass vs. urethane?

I'm not an autobody expert, but what i can tell you is that both materials have their good points and bad points. Drive your car hard, bottom out a little? Want Durability? Go with Urethane, I've taken it and bent it to oblivion, and it just bounces back to shape. As far as I'm concerned it would take alot to break it. Fibreglass is light, as is urethane, but with Fibreglass it's easier to modify. If you're into that kind of thing, or know someone that is, it's pretty easy to mess around with fibreglass and add to or subtract from it. My recommendation is urethane, it's what all the big name companies are using, so it's gotta be doing something right

If you've ever looked at some of the ricer body kits that have been driven around for a while you'll notice that they are cracked/chipped on the bottoms of the skirts and front facia. That is the fiberglass. If you are going to do fiberglass only do it for upper body parts (hoods, hatches, trunk lids, etc.) it's great because of how light it is. If you are talking about doing parts that may at some point through no fault of your own come into contact with speed bumps or large pieces of debris in the road, you want Urethane or ABS plastics because they won't break and many times your paint will adhere well enough to them that the paint won't even crack or come off during these events.

You need to consider the climate in your area also, i live in canada and urethane is good and all but with the constantly changing temps, urethane would warp, i have friends with urethane kits and they are happy with them but in very warm weather urethane can warp. Fibreglass is good but only if it is a good quality, Hand laid fibre glas is better than a sprayed application and also better than a mold application. So if you want fibreglass make sure that it is of a good quality (hand laid).. you can tell what it is by looking at the inside part of the kit....if you see outlines of pieces of fibreglass cloth...then your good to go...but if it is all little tiny bumps then it is sprayed....good luck