heavy duty steel storage sheds are built with double wall panels to resist dents. The panels are made to exacting standards for a snug fit and solid construction. Many come with paintable steel doors for extra security and long service life. Best of all, the entire structure is impervious to practically all weather extremes and virtually maintenance free.
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.