I've got a couple of fiberglass/plastic ones mounted behind each of my back dually wheel sets. They are made by Husky, look good, moderately priced, and are durable.
I also have been wondering which style are best. Anyone have any experience with the brush style? Do they let the wind through better than the solid rubber ones which I have seen really get blown backwards by the wind? In this position, they don't offer much protection.