From what I've read, aluminum cases do practically nothing for the heat. The benchmarks I saw showed an insignificant decrease in temperature. So pretty much aluminum is only good for LAN parties since it's light.
yes,.. the only reasong to take an aluminium case is the weight. altough.. the material isnt as strong as (good quality) steel,.. so you can damage a case a bit easyer.
as for the heat,.. it doesnt really matter, its the design of a case that allow's it to blow the heated air out much easyer,.. the case actually shouldnt absorb any heat
I don't know the material my room mate's case is right off, but another room mate commented about me not putting a standoff in the center while I was building the computer(motherboard was flexing, BAD) It turned out to be the whole case flexing! It was just a cheap < $20 case