When I got my FP knees I thought I was getting fiberglass, but Chris told me he wasn't going to do them in fiberglass anymore as unfortunatly fiberglass ones can crack too and the gel coat can flake off thus ruining the paint job, that is why FP decided not to go with the fiberglass knees and went with styrene. Styrene has a little give to it where fiberglass dosn't and can still crack and break.
Not only that, fiberglass is heavier and will be harder to stay up then styrene.
Besides, I beieve the real ones are made from styrene, if it is good enough for the real one, it is good enough for me. 
Lynn