Urethane parts are made of reaction injection molded polyurethane, a material that is lightweight, flexible and resistant to rough weather conditions and everyday road hazards. Because of its flexibility and light weight, parts made out of urethane are usually easier to install. There are several companies for manufacture car body kits using high quality urethane, all are very 'reputable' in brand name.

Fiberglass is a material well-known for its resistance and rigidity. It is crucial to choose the highest quality fiberglass for your body kit. Many companies create bad quality parts containing wavy patterns on the fiberglass and unsatisfactory fit. Fiberglass components that are hand laid have no waves and are not brittle or weak. When a part is hand laid, the part will be uniform and will have an even amount of fiberglass throughout the component. Hand laid fiberglass takes the longest, and costs the most but is worth it. There are a number of companies who make reasonable body kits using high quality fiberglass. However to this day, I don't know of any companies that offer high quality hand laid fiberglass kits for standard cars such as the VW golf, Corsa, Ford focus, etc..

Below are some of the major differences between the urethane and fiberglass.

Urethane kits are flexible so any slight impacts or bumps will not crack the kit. Urethane kits are resilient to weather so hot or cold, they can withstand the climate (rain, hail, snow). Urethane kits are easier to install since they have some flex to contour to the body parts. Urethane kits are easier to prep before the painting process thus costing you less at the bodyshop.

Fiberglass kits are not flexible so the slightest bump and she's cracked. Fiberglass kits are not resilient to weather so they can get even damaged even easier due to this. Fiberglass kits are difficult to install because they almost always require cutting, sanding down, etc to actually get the part to fit properly. Fiberglass kits are more expensive to prep and paint, due to the needed shop time to get the kit to fit properly.

What it comes down to in the end is you can 'usually' get a high quality urethane kit purchased, prepped, painted and installed for less then the cost of a fiberglass kit.

Another thing to keep in mind, No brand name body kit company out there (commonly) uses fiberglass to construct their kits. That should say something if you haven't gotten the point already.