I just talked to my friend who does jumping and Recomended the wooden jumps....
When your showing people use wooden jumps.
When your training you want the jumps to appear as much like the other jumps, if they nick it the nick will "feel the same as the jump at the show or in training" You want the jumps to Not appear different at a show then in your training sessions.
The idea is to not have the horse fear the jump!
I have found through experience that horses do better with wooden poles because when they hit it, it hurts more, teaching them to be more careful.