Hi Jason,
If you are purchasing Worthington steel cylinders you can look at the specifications for the tanks here: XS Scuba Worthington Steel Cylinder Specifications.
I like to look at tanks from two perspectives. The first is how they behave in water and the second is how they behave out of water.
In water you want to think about the buoyancy and the size. How the weight is distributed will depend on part on the size. A long, thin cylinder will distribute the weight differently than a short, fat cylinder. The LP121 is an 8 inch diameter and over 29 inches long. As a reference, an AL80 is typically 7.25 inches and 26 inches long. With a really long cylinder you might find yourself foot heavy or head heavy. If you centre the cylinder you could find it is hitting your legs. If you move it up to avoid leg hit, you could become head heavy. Only using the cylinder will be able to tell you how it will work for you. Two people 5'10" might have different weight distribution and therefore react differently to the same cylinder.
The other in water factor is buoyancy change. If I have a cylinder which will lose 10 pounds of weight as it empties, I must be 10 pounds heavy at the beginning of the dive to avoid being too light at the end of the dive. A cylinder which only changes 5 pounds in buoyancy would be better. Looking at the Worthington cylinders, you are typically looking at 6 to 8 pounds change. So this is not a factor when comparing two Worthington cylinders.
Also, if you are used to weighting yourself for AL80, the steel cylinders will typically be 5 pounds heavier in water. This means you can remove 5 pounds from the weight belt.
Out of the water there is the weight and the height. If you are short person or have long arms and no upper body strength, a tall cylinder can be a pain. My former instructor is short. She cannot carry two AL80s easily. She needs carry one at a time and tilt sideways to stop it from dragging on the ground. On the other hand, an HP100 is 2 inches shorter and she has no trouble carrying two HP100 cylinders. An LP121 would be monsterous for her to carry. The other thing is the weight out of water. An AL80 is 31 pounds. An HP120 is 38 pounds and an LP121 is 50 pounds. The HP120 is 7 pounds heavier BUT you dropped 5 pounds from your weight belt, so total kit is only 2 pounds heavier. Not bad. On the other hand, the LP121 is 19 pounds heavier. Even when you subtract the 5 pounds, you are 14 pounds heavier out of the water. This might not be a problem if you are boat diving as it is steps to the back of the boat. However, if you are shore diving and there are waves, this could really tire you out.
Finally, check with the agency you are training with. I have heard some agencies or possibly some instructors require specific gear. You don't want to buy HP120s only to find out the instructor insists on HP130s.
