Definitely the aluminum is much lighter. Back in the day you were expected to carry 2 rolls of bx, one on each shoulder, up to whatever floor needed it. MC lite, no problem.AC cable is harder to work with because the sheathing is tougher, and there are more restrictions on places where it is allowed. I bet it costs more to. Around my fair city nobody stocks AC so if you wanted to use it it would be special order.
Its partly a geographical thing. Some places I have asked the guy at the supply house doesnt even know what AC is and its not stocked. Some places its an everyday item. IT does seem that overall MC is much more common. I dont know why that is.