Particle board and plywood are composed of pieces of wood products that have been pressed and glued together. Although they are popular siding materials for storage units, they are not most people's first choice in regions where it rains a lot. For if water does work its way into these products (a possibility enhanced by their composite nature), then you're lost.
Due to their susceptibility to rust, steel storage units should be avoided. Among metal storage units, rust-free aluminum is usually the most sensible choice in siding. Metal siding is hardly the most visually appealing choice.