"It depends."
Not very helpful, but that's really the answer. If you don't mind the excessive weight, couldn't go wrong with 1.5mm steel from a strength perspective.
But like I said, it'd be on the heavy side. 1mm or slightly under for steel is good for strength/weight but without stiffening bends, a flat piece of 1mm steel is surprisingly flexible if it has large cutouts in it. Plus steel can't be anodized.
1.5mm would probably be the minimum thickness I'd use for aluminum though except for particularly small cases. But most manufacturers have trouble cutting aluminum thicker than 2-3mm with lasers so 1.5mm is a good conservative option if the frame is designed with plenty of reinforcing bends to stiffen it.
But if the shop has water jets that they're pricing reasonably or a state of the art laser cutter then the shop may be able to easily deal with thicker aluminum.
As you asked in your PM, using different thickness of metal is fairly common. Often the side panels will be thicker since often they're a mostly flat piece so the thicker metal can help keep the panel from flexing to much compared to if it was made of the same thinner gauge as the frame.
But in general you want to minimize the number of different thicknesses and types of metal used to reduce manufacturing cost. If the shop can cut out all the parts out of single sheet of stock metal that's cheaper than if they have to cut out some parts on one sheet, swap for a different gauge and cut some more parts, swap yet another sheet and cut a few more parts, etc.
