Well I'm looking at two cases right now, the CM Centurion 530 and the Tt Tsunami. One is steel and one is aluminum. Apart from that, they're quite similar. The Tsunami has a window and an LED rear fan, and the Centurion has top-mounted I/O and side intakes. Those are the parts that I like about one case that the other doesn't have. Anyway...
Does it really make that much of a difference if I go with aluminum? I'd prefer the sturdiness of steel, but not if it means huge heat increases.
The difference in temperatures between steel and aluminum, in our tests, has shown to be less than 5C. Steel also has the advantage of being a much sturdier material, and dampens vibrations a lot better.
Aluminum is the only way to go nowadays, owing to its lightweight structure and better heat-disappating qualities. Steel is a thing of the past.