[ger21]

For most people, wood is cheaper and easier to work with than metal.

As for MDF vs plywood, MDF is much more stable. Unless you use good quality baltic birch ply, most plywood sold today will warp severely if not cut and assembled within a day or 2 of being puchased, provided you can find flat plywood to start with. If you seal and paint your MDF, you really shouldn't have any moisture problems.