whats different between those two?

and which type is better for bodykits?

 

Urethane is heavier but has more "give" to it. It is flexible.. you can bend it. Fiberglass is lighter but more prone to cracking if you hit something because it really doesn't have enough flexibility to withstand a good impact, it won't bend like urethane does, whereas urethane will generally bend with the impact and return to its original shape. I prefer urethane because of its durability and because its flexibility generally makes it easier for a body shop to work with if they run into any fitment issues.