Fibreglass, carbon and PU.
In terms of quality it really depends on the the manufacturer.
We personally prefer very low front lips to be made out of PU because it can take much more abuse then fibreglass and carbon because they have a lot more rebound and less brittle.
However again depending on the manufacturer. We've helped fitted many lips made in PU and most people's impression is that PU is fantastic in fit but that's not true.
Fitment can only be good if the mould and product out of it is made well but being made of PU you do have the ability to heat and re-shape certain edges which you can't do with fibreglass and carbon.
One other thing about PU is that there are many rubbish productions out there being made so thin that they droop and sag. As you can imagine, at speeds this causes it to droop and drag on the road and I've seen many being shaven away.
Worst case is if they're so droopy it's impossible to paint because the hardened paint will just crack away where it flexes so much.
The quality between fibreglass and carbon again is dictated by the finish of the fabricator. Most fibreglass products are finished in a raw or primed form as the body shop is expected to do the prep and paint work for colour matching.
We offer the choice for a black gel finish on certain parts so that painting isn't required and can be fitted straight on.
In terms of strength, the carbon is much stronger compared to fibreglass with the same density and a lot lighter. Hence the cost I suppose.
But remember with carbon, though stronger and more lush, the top coat is of a shiny brittle resin which cracks easily.
Something to think about when choosing.