I've never had anything but wood......so I would say wood.......LOL
I always was told that if you stagger the nest front openings, there's less chance of the birds getting confused about which box is there's. You can also put numbers or letters on the doors OR you can tack playing cards in the back of the box. The birds DO recognize such things. That's why our nest fronts in the widowhood loft are painted different colors.