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.
The small touches make a difference. Some outdoor storage buildings come with French doors or cupolas. You, yourself can add touches such as window boxes, shutters, or weathervanes. Remember, you'll have to look at this outdoor storage building every day of your life for the foreseeable future: Your attention to detail could make the difference between eyesore and eye-opener. On a more practical note, purchase or build a pressure-treated ramp for easy access to your outdoor storage building.