R&G's are good, i have them on my bike. Not tested them yet though
As for materials, the metal will slide a bit better along the ground perhaps, but there is a risk that they can snap a little easier (metal dosnt have as much elasticity as plastics). I think mine are nylon, and dense nylon has good strength and abrasion properties.
Titanium ones would look best in a crash, all those sparks
Matt