ok so a few points...
for the sake of these comments i'm going to assume you would use 11 or 13 ply birch plywood, not the cheaper grade stuff.
plywood and mdf are both going to react with their atmosphere - humidity to be exact. BOTH must be sealed to be stable. plywood will warp/bow and mdf will flare at the edges.
plywood is much much MUCH stronger (tensile and compressive) than mdf. there is a reason no boats are made of mdf (and it's not because mdf isn't waterproof). however, mdf is more rigid. there are some things that could also benefit from plywood providing better dampening and strength.
a popular sealing method for plywood is very low viscosity epoxies - a few coats of this stuff and your plywood is crazy strong. you can do the same on mdf but you have to be very careful not to overapply and saturate the material. mdf is much more porous than plywood. my opinion is that it's better to seal mdf with a barrier coat (like a paint or primer) than a penetrating sealer (like diluted epoxy).
mdf is an engineered material that is effectively flat on it's surfaces. plywood is not flat, even the very best stuff. also, mdf machines very nicely and uniformly.
plywood weighs approx. 25% less than an equivalant amount of mdf. heavier does not always mean better/stronger...
at the end of the day, plywood is a really amazing product that outperforms many other materials on a pound-for-pound basis. however you have to buy the good stuff and that gets expensive, and it's harder to find. also, you loose the close tolerances that you get with mdf. you could make a router completely with mdf, but it would be tough to make the same router with the same tolerances using plywood. HOWEVER - you could make a stronger and lighter router using a mix of both!  |