What is the difference in the types of body kits like fiberglass and plastic, and ect. like which 1 is more durable?

urethane is more resistant to cracking than fiberglass

^

I think he hit it on the nail. This info is from my own experience with the two (which is limited by the way, but I have bought/used both types)

Urethane:
Ups:
Less likely to break
Cheaper (fitment/paint can raise costs though)

Downs:
More prone to warping in high temps
Fitment(due to warping/poorly made products) & painting costs

Fiberglass:
Ups:
Cost less for paint
Usually fits better because it doesn't warp as easily as urethane
More sturdy (doesn't warp ^)

Downs:
More expensive
More likely to break

Hope this helps a little,
Will

though urethane is less likely to crack. when it does, you basically have to throw it away.

weight of the parts also is different. fibreglass is a bit heavier.

^ +1 :) i would prefer urethane than fiberglass ;)