An Al80 is an ALUMINUM tank that holds 77 cubic feet at its rated pressure, 3000 psi. An HP80 is a STEEL tank, which holds 80 cubic feet at its rated pressure, which depends on the maker of the tank. Several manufacturers make a "high pressure" steel 80, but the working pressure varies, as does the pressure at which the tank contains its stated volume.
Aluminum tanks are relatively inexpensive and are not as subject to rust, but they impose a significant weight penalty on the diver, as they are both physically heavy for their volume, and also require about 5 pounds or more of additional lead to sink them, compared with steel tanks. They are virtually ubiquitous in tropical resorts, where corrosion is a big issue and tank expense is, too. On the other hand, steel tanks are almost universally used in cold water, because of the weight considerations. Steels are more expensive, and can develop problems with internal (or external) rust, if not handled properly.
In buying a tank, you need to look at your budget, your physical strength, the type of diving you are doing, and your size. HP80s, for example, are very short tanks. If you are quite tall, putting a heavy steel object high up on your body may make you tend to go head-down all the time. 120s are VERY long tanks; I cannot sit down while carrying one. They are best used by tall divers with long torsos. HP130s have a ton of gas in them (enough to get into trouble
) and they are a reasonable length, but they weigh almost 50 pounds when full, which doesn't make them fun to move around on land -- and because they are only about 1.5 pounds negative when empty, you don't get to jettison any weight by using them.
Recognizing that this is my own personal opinion and nothing else, I think, for cold water diving, HP80s work great for small women. LP95s are inexpensive but heavy, but have enough gas (with the 10% + rating) to do almost any recreational dive above 100 feet and in cold water. HP100s are delightful tanks, and are expensive and hard to find because of that. They're small, relatively light for their capacity, and hold enough gas for almost any recreational dive. HP130s are what you use when you don't want to dive doubles . . . you can easily get into deco using them, which is not a terribly good idea on a non-redundant setup. 130s are perfect for scooter dives in Monterey
Tanks should be very low on the acquisition list, because they are easy to rent, and around here, most shops rent them for only a little more than the fill price.
