The pendulum keeps swinging on which cylinder is the safest to use. SCUBA cylinders started off being manufactured with steel, which was the standard material used by the industry. Eventually, issues began to be noticed with corrosion in and around these cylinders. The corrosion became a concern and people wanted to reduce the risks. It is darn near a guarantee that when you combine Steel with oxygen and water (all components in diving) corrosion will soon follow and the risk of rupture while filling a steel cylinder will increase.
In the 1960's aluminum started making a presence in SCUBA cylinders. The U.S. Navy started using aluminum cylinders with some of its divers for its non-magnetic properties. The NAVY had questions about the safety of its cylinders, both aluminum and steel, so they began a testing process. In 1970 the U.S. Navy conducted a study of their cylinders and issued a report. In the report the NAVY stated they found "no significant safety difference between the steel cylinders and the aluminum cylinders." With damage, any type of cylinders can rupture. However, with proper maintenance and the ability to detect damage, the risk of high pressure cylinders is reduced.
Then in the 1980's cracks were found in some aluminum cylinders manufactured with a 6351 alloy. The word of these cracks spread with the help of the internet, agencies and some people called the cylinders unsafe and a hazard. The facts did not follow the information. The U.S. Department of Transportation (DOT) stated In 1999, of the estimated 25 million cylinders made of 6351, only 12 were reported to have ruptured. Even though this only equates to .000001% of the cylinders, people were afraid to fill or use them.
Since that time aluminum cylinders have started to fall from favor. Divers have started switching back to steel cylinders, assuming they are safer. Divers state that they enjoy the negative weight (still negatively buoyant when emptied) and the durability of a steel cylinder. However, the corrosion concerns when steel is combined with water and oxygen are still present.
The pendulum will keep swinging between these two metals until a third option becomes available. Some composite cylinders have been approved for underwater use, but they may have their own weaknesses. There is no problem with someone preferring a specific characteristic of a type of cylinder. One type of metal is not safer than another. They simply have different qualities. Diver's should choose a cylinder on price, size and specific qualities and not base their decision solely on the type of metal.