When you travel abroad, other than by car, you get little choice in the type of cylinder you use, and this could make you indifferent to the possibilities open to you. But when you are diving in your home territory the choice available, although relatively limited, is worth examining.
It is worth selecting your cylinder carefully because it is the single component on which you are going to expend the greatest effort moving from place to place, on and off boats, in and out of car boots and up and down dive-boat ladders.
First consideration could be the choice of metal - steel or aluminium. Let's dispel the myth that aluminium cylinders are light. The characteristics of aluminium produce a cylinder with a much thicker wall than that found in a steel cylinder, so considerably more metal is required. An aluminium cylinder with the same water capacity and working pressure as a steel cylinder will be larger and heavier.
The fact that it is larger means that it displaces more water and so has less negative buoyancy. This results in some aluminium cylinders becoming buoyant at the end of the dive. Logically, because the mass of air consumed is the same, the weight difference will be the same.
The problem, particularly with US-made aluminium tanks such as you might meet in the Caribbean, is that they start the dive negatively buoyant and end it positively buoyant. A harness arrangement that works well with a sinking cylinder will not necessarily function well with a floating cylinder.
The choice of working pressures is generally 232 or 300 bar. The former is universally available at charging stations, 300 bar is not. This makes the choice of capacity interesting, because a 10-litre cylinder charged to 232 bar (2320 litres) gives only 10 per cent more air than a 7-litre cylinder operating at 300 bar (2100 litres).
The 7-litre cylinder is lighter out of the water, but will be more negatively buoyant in the water, because of its lower displacement.
The slim 7-litre cylinder makes a nice compact twinset, but with less total capacity than a single 15-litre cylinder (all at 232 bar), and costs a lot more.
Why are 10.4 and 12.2 litres popular sizes? Simply because they were introduced during the metric conversion phase and were the equivalents of the 80 and 100cu ft cylinders of the day.
There is no difference between a 12 litre and a 12.2 litre cylinder. They come out of the same manufacturing process and are stamped up in accordance with the customer's wishes, because this 0.2 litre difference is well within the manufacturing tolerances.
If you find you always seem to run low on air before your buddies do, you might be wise to treat yourself to a larger set and breathe away to your heart's content. This is better than skip-breathing or dragging your air out to avoid being the one to signal the end of the dive.
If you want a cylinder for use with nitrox, look for an O2-clean specification. It is far more economical to clean the cylinder during the manufacturing process than to do it later. It will also need to be fitted with an oxygen-compatible valve.
If the cylinder is offered to you as new, check the manufacturing date stamped on the shoulder. If it is well past this date the need for the cost of a re-test will arrive sooner.