I'll add one more thing to the equation. The HP120 tanks are tall. When you have to transport them, they're tall enough that when you carry them, you'll need to have your arm at a funky angle to keep them from dragging on the ground (unless you're 6'8" tall or something, or have midget short arms). Moving them anything more than fifty feet and it gets annoying. I have one that I use for So Cal boat diving (mainly because of the short fills and sometimes wet fills on the boats) and that is my biggest complaint. Underwater I don't notice the length at all (I'm 5'10" tall).
Stick with the aforementioned HP100's. Practically the same size as an AL80. The 8" tanks (LP95, LP108, HP119, HP130) are big enough that they don't fit in standard tank holders on the boat.