I like MDF because it is so inert, like a rock. But it isn't very strong really, you can chip edges pretty easy and it has very poor water resistance. Soaks up humidity like a sponge and grows like one too. Plywood is stronger, but you have to find good stuff that doesn't have voids. It's not really the voids I'm worried about so much as it is the debris that can be caught inside the void. That and the possibility of two laminates buzzing together.

One other thing is the use of fasteners. If screws are driven into plywood edges, they will separate the plies and make a void. You can get away with this on solid hardwood stock, but not plywood. If a screw must be driven into a plywood sheet on edge, a pilot hole should be drilled first, so that the shank passes easily and only the threads bite into the wood. That will help prevent separating the plies.

Now, with the new thin drywall screws, don't really need the pilot hole in the sideways/bottom piece. Much better clamping, too. The best screws now are Spax, made in Germany, not too expensive, designed for wood, but kinda like a drywall screw, have *really sharp* serrated edges on threads, need no pilot holes- just line it up very straight and clamps like a demon.
The ply at Home Depot with the birch or oak outside layer is very good. This new stuff they have, called Sandex or something like that, is not.

• Spax Screws