I guess ply was the best material way back, and MDF is a pretty recent (comparatively) material. The stuff has been banned in the US I believe for quite some time due to poisons used in the manufacture. Given the way it fails to do its job all that well  in the building side (put a drop of water on it and watch it expand and turn into crap) I can't imagine wanting to use it in a pinball (not that water is used, but you wonder what some Novus 1 might do). I think ply due to its construction has a tendency away from warping.