The choice of scuba cylinder material is ultimately a personal factor, although there are informed logics that can help determine that choice.
The biggest influencing factor is the diver’s buoyancy and weighting needs.
Cold water divers typically utilise dry suits as exposure protection. Dry suits are typically very buoyant and demand a considerable amount of ballast (weighting) to sink.
Drysuit buoyancy also remains constant regardless of depth (because more gas can be added to compensate for compression), whereas thick wetsuits will compress substantially at depth – losing a large amount of positive buoyancy and, thus, leaving the diver utterly reliant on their BCD to offset their weighting.
Rather than add large volumes of lead weights to their harness/belt, many drysuit divers opt for steel cylinders as they’re more negatively buoyant. As such, the cylinders become a part of the diver’s weighting strategy.
This is why you’ll hear many cold/temperate water divers automatically suggest that steel tanks are better. However, in doing so, they’re showing inexperience in the demands of other environments and/or a lack of safety-focused consideration on diver weighting.
A diver was using a wetsuit might not need so much ballast, or may consider it prudent not to resolve most of their weighting need through an item (the cylinder) which couldn’t be jettisoned in an emergency.
For the wetsuit diver, steel tanks would potentially replace too much lead weight – meaning the diver had too little jettisonable ballast should they experience a buoyancy device (BCD) failure. They may become too negatively buoyant to swim unassisted towards, or float at, the surface – which is, obviously, a major safety consideration if you don’t want to drown.
The drysuit diver, on the other hand, has an inherent solution for buoyancy device failure – they can use their drysuit itself to provide the buoyancy they’d need to surface safely.
“Old school” divers who persist with using heavy steel cylinders with wetsuits usually compensate by equipping with very large capacity BCDs that have a redundant back-up bladder. These are more bulky, more complex and cluttered… and more expensive. It’s merely a matter of old dogs not learning new tricks i.e. not keeping informed of contemporary, proven, best practices for safety and performance.
The concept of intelligently considering cylinder buoyancy characteristics as an integral factor within the overall safety performance of the diver’s kit and configuration is known as the “balanced rig” approach. Unfortunately, many of the larger scuba training agencies neglect to teach that logical approach.
A secondary factor influencing cylinder choice is simply the availability of cylinders. Steel tanks are ubiquitous in cold and temperate water regions.
In contrast, you will rarely find steel tanks in tropical areas – aluminium is used almost exclusively. This is because, as mentioned, warm-water divers rarely need, or want, very negatively buoyant cylinders. Also aluminium is more suitable for hot, humid climates where oxidation is a bigger issue.
A third factor would be the gas supply demands of the diver. Aluminium cylinders are rarely available in capacity greater than 100cuft/15-litres, and are limited to 200bar/3000psi pressure. Steel cylinders are available in greater capacities; up to 20L volume and pressures of 300bar/4500psi. Thus, steel cylinders can supply the diver with much more gas.