Type of Material
Aluminum tanks have a thicker wall and tend to be more susceptible to dings and scratches. Steel tanks have a thinner wall and tend to be tougher and more rugged.
The capacity of the tank will give you an idea of the approximate amount of compressed air that it holds. For example, an Aluminum 80 holds about 80 cubic feet of air (actually 77.4 to be exact).
The pressure rating of a tank does NOT indicate how much air it holds, in otherwords, a higher pressure tank does not necessarily hold more air. The pressure rating indicates at what pressure the tank is considered full to its capacity. So an Aluminum 80 tank with a pressure rating of 3000 psi will still hold more air than an Aluminum 63 at 3000 psi. And a steel 95 with a working pressure of 2400 psi holds more air than both.
High vs Low
High pressure steel tanks are usually a bit smaller and more compact compared to their low pressure counterparts. Because they require a higher pressure (3300-3500 psi) to be "full" it can be difficult to get a good fill. The higher the pressure, the closer the air molecules are compressed and the hotter the air will get. A tank can get very hot to the touch while being filled to the maximum pressure. Once the tank cools off, the air molecules cool and the pressure in the tank will drop. For example, with a high pressure steel 80 tank, if it has less than its service pressure of 3442 psi once it has cooled, you will have less than 80 cubic feet of air in the tank. Getting a good, full fill on a high pressure tank is very difficult. This is one drawback of a high pressure scuba tank.
Aluminum tanks tend be become positively buoyant as they lose pressure, so you may need to add more weight at the start of the dive to offset the slight positive buoyancy effect that will occur at the end. Steel tanks tend to stay negatively buoyant throughout the dive.
A DIN valve requires the regulator to screw directly into the tank valve. This style is more popular in Europe and with technical divers. A yoke valve requires the regulator to fit over the top of the tank valve and then is held in place with a tightening screw. This style is more popular in North America and the Caribbean, especially with rental tanks. An International or Pro-Valve looks like a yoke style, but has also has an insert that can be removed with an allen key which then allows use with DIN regulators.
Care & Maintenance
When storing or traveling with your scuba tank, ensure it is safe from getting banged around or knocked over. Keep it safe from scratches, dings, nicks that could damage the integrity of the tank. A visual inspection by a certified tank inspector is required every 12 months to ensure the inside is clean and free of debris, the neck threads are not damaged, and the valve is functioning properly. A sticker on the tank will indicate the most current visual inspection completed. A hydrostatic test is required by Department of Transportation every 5 years and is indicated by an engraved stamp on the tank near the top.