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.