Steel tanks weigh less, are smaller and more durable, and require that a diver use less weight than standard aluminium tanks. Why aluminium tanks have become the industry standard then?
Aluminium is more cheap material, it doesn't tend to rust like steel, so requiring less inspection efforts.
Aluminium is less strong then steel, so AL tanks walls are more thick to withhold the pressure of 200 bar.
Steel tanks are slightly more compact.
You'll need more weights for diving with AL tank - it tends to float when empty, while steel tanks are always negatively buoyant.
Aluminium tank 12L - when full — 0,7kilo (negative buoyancy, sinking), when empty +1,8 kilo (positive buoyancy, floating),
Steel tank 12L - when full — 4 kilo (negative buoyancy, sinking), when empty — 1,3 kilo (negative bouyancy, sinking).