I think steel might be more rigid of course, depending on the sizes and types used. However, I think that either material is well suited to do what you want.
I built my first small router using steel tube. It is rigid. I didn't weld anything but the lower shelf onto the legs. The rest was all either drilled and tapped or through bolted. You could easily cut all your parts with a hand held bandsaw. They are cheap enough.
I built a small mill using the 3060 as a base and column. It is very rigid as well. It is much easier to modify an 8020 design than a steel one, but given the steels strength, I would mark them as near equals with the edge going to steel.
You can save a lot of money on your design as well. Use only 3060 for the gantry and the sides of the table. Use steel or aluminum tube for the rest. Make your own corner braces and brackets using steel or aluminum.
You can make it incredibly strong and cheaper this way. You would be able to over brace for less money.