Agree completely w/ what Bill writes. 2 additioanl points; neoprene is a lot safer than rubberized canvas/nylon as it will take less water if you flood the waders in since it is a tighter fit.
Second, whatever you end up getting make sure you get a belt to go with it. A blet limits the amount of water that will pour down the waders if you flood them.
Also - its probably a good idea to experiment with flooding waders in a controlled situation so you understand what sort of mass and bouyancy you do/don't have.
Its a terrifying thing to be pinned in the water by the weight of flooded waders.