Fibreglass bodykits tend to be a bit brittle. However, most body shops are familiar with the material and can easily be repaired. Urethane is more rubbery in nature, better able to absorb shocks better. Not sure about weight differences between the two.Most original body kits are made of ABS plastic, which tend to hold shape better and not as brittle as fibreglass.Unfortunately for me, bodykit for e21 are hard to come by. So I had to get mine done by BumiFibre. FYI, the finishing quality of Bumi Fibre is very poor. Luckily for me, my painter is a cool guy and rectified it for me at minimal additional cost.If you can still get original stuff for your bimmer, bite the bullet and get it!