Check the capacity. More air is better, of course, but you’ve got to have an intelligent balance between capacity, physical size, weight and buoyancy. Bottom line: there is such a thing as a tank that’s too big.
Aluminium V Steel
For similar volumes both tank weigh fairly similar but you need to carry considerable more lead with Aluminium. Aluminium tanks have the advantage of been cheaper and do not have the inherent risk of rusting a steel tank does. The rate of failure from their annual test is much higher with a aluminium than a steel. Personally I much prefer steel.
Choosing a tank that fits your size, air consumption, buoyancy and style of diving can make a world of difference on your diving enjoyment. Don’t rush into buying a tank until you know what you want.