I have steel fenders on my Cobra trailer. *A blowout took the back
half of the fender and folded it double inside itself. *So I am not
sure steel is any more durable than plastic. *

Really? Imagine what your fiberglass/aluminum trailer would have
looked like if that steel fender hadn't absorbed like 90% of the
energy of the blowout. In addition to a mangled trailer, your glider
would have probably been damaged inside as well like this recent
victim of plastic fenders:
http://groups.google.com/group/rec.a...9415b95aad7d6#

Another good reason for metal fenders is that they provide a little
more protection from other cars/trailers as well...

-Paul