Steel rusts and aluminum does not. (Aluminum does slowly corrode, however.) Steel canopy poles can be treated or painted with products that protect them from damaging rust.
Steel is heavier, and therefore more stable than aluminum in windy conditions.
Because aluminum is lighter, it is possible for just one person to set up the canopy.
Steel frame canopies and aluminum frame canopies cost about the same.
No canopy is safe in the wind. All canopies, both steel and aluminum, need to be staked or weighted down in the wind.
Aluminum bends more easily than steel. An aluminum canopy that travels to fairs and is often set up and taken down may need replacement parts more often for this reason.