Aluminum cylinders are popular because of their lower cost. Another advantage is that aluminum doesn't rust. But these cylinders also have some disadvantages. Aluminum is softer than steel, so they can be dented when dropped or knocked over. Also, the cylinder walls must be thicker to compensate for aluminum's weaker strength. This results in a larger physical size than steel cylinders of the same capacity. Aluminum is also more buoyant than steel, so you'll need to wear more weight when using aluminum cylinders.