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.
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.