I did compare apples with apples. More precisely I compared tanks that hold similar volumes. If I want an 80cu.ft. tank the HP version is the lighter of the available options. Claiming that HP tanks are heavier because an HP133 weighs more then an LP108 is not comparing "apples to apples". I don't know how everybody else plans dives but I don't do it based on the length or diameter of the tank, I do it based on the volume of gas the tank holds, and for similar volumes of gas the high pressure tanks are lighter.
You did make very good comparison, and I agree with you when comparing the same volume, but you can not please everyone because somebody will throw the high pressure fills in a LP cylinder, or the life expectancy of tank, or etc, etc. This comes down to how you compare the tanks.
Just adding to your comparison of same volumes, but with Faber cylinders using the about the same volume, trying to hit 80 cubic foot.
HP80
Rate Volume: 80 cubic foot
Internal liquid Volume: 622 cubic inches
Length: 20.87 inches (not including valve)
Weight: 28.6 lbs (not including valve)
Buoyancy: -8.05 to -1.74 lbs (not including valve)
LP85
Rated Volume: 85 cubic foot
Internal Liquid Volume: 793 cubic inches
Length: 25.98 inches (the diameter of this tank is 7" not 7.25")
Weight: 31.2 lbs
Buoyancy: -3.8 lbs to 2.32 lbs
LP80 (Faber does not specifically make an LP80 under the DOT specification cylinder, but they do have a 80 cubic foot cylinder rated at 2,640 psig under the exempt cylinder specification, FX100LPDVB)
Rated Volume: 80 cubic foot
Internal Liquid Volume: 787 cubic inches
Length: 24.80 inches
Weight: 24.9 lbs
Buoyancy: 1.65 lbs to 7.65 lbs
Luxfer AL80 (XS Scuba)
Rated Volume: 77.4 cubic foot (I could not find the liquid volume of an AL80 tank)
Length: 26.1 inches
Weight: 31.4 lbs
Bouyancy: -1.4 to 3.4 lbs
When doing this comparison the HP80 is lighter, and smaller than the LP85 and the AL80s, but are also 6" shorter too. The height of the HP80s can and will affect the bouyancy and trim of a tall person. I can not dive HP80s because they put me in the head down position, and that is the worst position to be in. HP80s are good for some, but not so good for others.
When doing this comparison, this is where almost everyone will pick the LP85s over the HP80s or the AL80s. I left out the LP80s under the exempt specifications because those cylinders float when full, who in their right mind would buy those cylinders? Why do you want to carry even more weight? I also left out the mid-pressure cylinders because those cylinders are bricks, -12.5 lbs (full) to -6.58 lbs (empty), again who in their right mind would buy those cylinders.
My favorite cylinders are the HP100s. When I need to get larger cylinders, I will get HP133, because they are the HP version of the LP108.
HP100
Rate Volume: 100 cubic foot
Internal liquid Volume: 787 cubic inches
Length: 25.39 inches (not including valve)
Weight: 34.3 lbs (not including valve)
Buoyancy: -8.41 to -0.59 lbs (not including valve)