I'll make it another vote for plastic. Aside from the reason cited by the eminent Mr. Kabbaz, sooner or later a collar stay is gonna go to the laundry and get laundered with the shirt. I think the brass can cause discoloration then. In event, collar stays are easily lost, and the cost of replacing plastic is negligible. Somes stores will give you a little package or two of them free at least if you buy a shirt.