On my soapbox, here . . .
Aluminum: Myth: "The higher thermal conductivity matters." [Only if airflow and cooling is so inefficient that you need to consider the case as heatsink.] Truth: "They're much lighter." Truth: "they CAN BE prettier (without paint.)."
Steel: Heavier. But also -- stronger. You can tap holes in 1/16" to 3/32"-thick sheet metal, and feel confident that the screws won't eventually strip the threads.
Nothing wrong with a steel chassis fitted with aluminum shroud and side-panel.
My bias: I've done most of the work needed to mod a '94 ProLiant Server case -- all steel -- some aluminum parts. I love it. With the experience gleaned from that, I have two full-tower ATX cases that are going to be given a real make-over. They're steel, also.
The b**** is in the painting.