IMO, rather than focus on material, I think the important thing is coverage and the mounting stays. First, you want fenders that are wide enough to cut down spray and not be a hazard as stuff gets sucked underneath the fenders. Second, you want to make sure the front fender is long enough on the aft side of the wheel to stop spray from kicking up onto your feet and drive train. You could install a mud-guard if the front fender doesn't provide enough coverage. Finally, you want fenders with good stays and a good mounting system, like the Berthoud fender and stay system (http://www.wallbike.com/berthoud/ssfenders.html). Bolt-through-mounting fenders such as these eliminate rattling and cracking, etc. Good luck!