IIRC JZ on a Brewstrong talked about this, and said it really doesn';t matter one way or the other. I keep my Co2, inside the kegerator and depending on what the welding shop has, get either aluminum or a steel tank. I've enver noticed any difference between the two.
At our Club meeting last weekend, there was quite a debate about this subject. I just kind of stayed out of it, as many had very strong opinions, and stated things like if it's in the fridge, you don't get all the C02 out of the tank and such. Or that steel should be outside, and aluminim inside, etc, etc.
Anyone have any good strong advice, or evidence on this subject?