it really depends on where you live. I heard polyurethane tends to warp if you live in drastic temperature changes. I live in Rhode Island where the summer sometime reaches 100 degrees and the winters go low as negative 10 degrees. I also heard paint doesn't stick very well on polyurethane. I have a couple of friends who had polyurethane add on body kits on some of their vehicles and the color on the kit changed tone after time.
However, the fiberglass kits and the polyurethane kits that Kaminari makes supposedly are very good quality. They claim that their polyurethane kits never change shape or form, even in extreme weather. I personally own the fiberglass version of their kit and I could vouch for their product. The paint looks great and still looking sharp. Hope this helps!
I live in Oklahoma, gets desert hot, arctic cold...but my baby will never see any of it in the garage. Kaminari will tell U that installation should be done by a trained pro, if the paint is done proper, won't be a problem. Kaminari also gives life-time warranty for any bending or shifting. So long as I buy from Kaminari USA, I should be good to go.