Aluminum is lighter and cheaper than steel. Steel is stronger, more abuse resistant. What are the conditions of use at your installation? Unless the engineer specifies steel, 99% of the time, aluminum will be chosen because of the lighter weight (easier installation) and lower cost.
Steel flex is still flexible and not rigid. Rigid is schedule 80 PVC, schedule 40 PVC, heavy wall metal conduit or intermediate metal conduit. Even though Electrical Metallic Tubing (EMT) seems to be rigid, nowhere is it refered to as a rigid conduit.