urethane.... strong, yes... but it doesnt hold paint as good. also, it doesnt really matter but it weighs more.

quality-made fiber glass should be the best bet.... as it's pretty cheap and easy to paint.

abs plastic is real good too but they are pretty expensive. my oem honda kit is made out of the abs plastic. my greddy lip is some blend of urethane/abs plastic. yes, i HAVE scraped now few times... :P

good luck. maybe wingswest has a lip or something.