I live near Austin, Tx so there are lots of wanna-be bands that start and then fail so there is almost always somebody selling off the CO2 tanks they have tried to use for the "smoke" so that leaves a lot of used tanks of all sorts showing up on the local ads. So I just buy up the cheapest used and do the swap at the local welding gas shop. They do not care whether it is steel or AL as long as it is in decent shape.
Two ways to look at which is better, perhaps. The aluminum can look nicer if that counts big but for lots of folks the tank is just another tool and how it lasts is more important. Where the tank just sets somewhere and pushes gas, the material matters very little but if you are doing rough work and dragging a tank around, steel is much more durable. Lots of tanks get thrown in the back of a truck or dropped off a roof so steel fits them better!
For our use, either is good.