I would say fiberglass has better fitment, and will hold paint better.

 

also urethane tends to sagg and warp over time.

 

like how long are we talking about here? Every eurathene piece I have ever owned never had that problem. I have seen urathene pieces 4-5 years old holding paint very well and in almost to new condition....can't say the same about fiber glass though.

 

It all depends on the quality of the urethane. Both poor quality urethane and fiberglass has its disadvantages. Fiberglass being wavy and uneven, and urethane sagging. Backyard Special urethane you would think is good quality, but ask some owners who have had the kit....very poor. Fiberglass done right is very durable and fitment is excellent.

 

fiberglass is not as britle as everyone is making it sound. my side skirt has hit a jack nothing happened my honda dealer hit my from lip on a curb no breaking. if you slam into something yes its going to break but not bumping into a curb.