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.
For nearly maintenance-free storage units, vinyl is the preferred siding. Vinyl lacks the visual appeal of wood, but if you own a house with vinyl siding, why not select a storage unit that is also sided with vinyl? However, vinyl will get dirty over time, making it even less attractive. Mold is often the culprit. Take into account the maintenance required to clean the vinyl periodically.