I'm in chicago also. I can't imagine them molding unless there was a
real water problem. But I would go with the plywood anyway. I think
it's easier to work with and more durable, even though it's lighter.
Particle board likes to crumble also. And I don't like how it takes
screws. I use melamine for the tops of utility shop cabinets I make,
but the rest of the box is plywood.