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.
Pay attention to job specifications, if there are any. In many specs, especially public works, steel flex is specified, and therefore aluminum flex would not comply. Remember, engineers do not have to install the stuff.
One other issue, there are cities in and around the Los Angeles area where approved types of aluminum flex conduit can be utilized as the grounding path, even when the length exceeds 6'. I believe it's 100' for 1/2" Al-Flex, less for 3/4", and less again for 1".
There are problems with the interpretation of this ruling, and it does not apply in all cities. Without saying this is a good or bad idea, many local contractors take advantage of the savings by not providing ground wires when installing Al-Flex.
There are 10 types of people. Those who know binary, and those who don't.
Radar, Is the Aluminum flex still OK as a grounding path in LA?? Somewhere I've got a "square cutter", a tool that was used to hold flex so it could be cut with a hacksaw. It seems like that was a requirement for using the flex as a ground. I haven't used it in about 20 years.