[toykilla]

urethane should be the best in my opinion.. would be lighter, and less resistant to cracking..

plain fiberglass will most likely crack, but is much cheaper than the other options..

frp is what most body kits (good quality ones) are made of.. it is fiberglass reinforced plastic.. so you get some flex and alot of stiffness.. good resistance to cracking.

carbon fiber is expensive.. but stong... you rarely see a pure cf kit.. but they exist