Originally Posted by chrism

what is with all the fiberglass hate?...here are some reasons why i chose my path in kits

Overview of the Two Materials

Urethane is flexible but very difficult to customize or repair in any way.

Fiberglass is rigid but can be readily repaired or customized by a competent shop.

Points to be Considered

Purchase Cost
Fitment
Ability to Customize or Modify
Cost and Ability to repair


PURCHASE COST

If you are purchasing a quality part or kit, the urethane products are probably going to be slightly more expensive.


FITMENT

Generally speaking, a urethane product will fit a car slightly better right out of the box due to the control in the manufacturing process.
However, a urethane product CANNOT be fitted as exactly as a fiberglass part. Fiberglass products usually require trimming or filling during installation.


ABILITY TO CUSTOMIZE OR MODIFY

A good custom shop can modify or customize ALL fiberglass products. Fiberglass is a material that can be shaped, changed or fitted to any application. These mods can be reinforced to produce significant strength and durability. Urethane cannot be reworked or modified with any degree of durability.


COST AND ABILITY TO REPAIR

Urethane parts are flexible and will absorb very small 'hits' without creating serious damage (cracks or holes). However, once urethane is cracked in any manner, it is usually sent to the scrap pile out back.

Fiberglass parts are totally repairable. Most damaged parts, within reason, of course, can be repaired by any shop experienced with fiberglass. Proper repairs on fiberglass parts are durable and should be permanent.

Let's look at a repair cost scenario on a full bumper replacement kit.
Suppose there are a couple of good sized cracks in the front bumper. Normally, with a urethane kit, the bumper will need to be replaced.
Usually, the part cost is approximately $425.00 + $125.00 (average) in shipping. The labor to remove/replace and paint will be a constant cost in either replacing or repairing. There will probably be a time delay of up to 4 weeks until the replacement bumper arrives. The same cracks in a fiberglass bumper can probably be repaired immediately for a labor cost of $150.00. In most cases, unless the damage is very severe, this means a savings of $400.00 to repair a fiberglass front bumper rather than replacing a urethane bumper.

taken from

http://www.wickedbodies.net/FAQ's.htm