A LP85 and HP100 are almost the same size, internally and externally. Thus at the same pressure, they hold the same amount of gas. The HP has the advantage that the worst fill you can get from an unknown dive shop is equal to the best fill your can get at the same shop with an LP85.
The HP weighs around 10% more, 31.2lbs versus 34.3 (Faber specs). But it's also 3 pounds less buoyant. Since you get to take those 3 pounds of lead off your rig, the net effect above water is 0 additional pounds. Well, call it 1 pound heavier because you are starting with an extra pound of gas.
An AL80 weighs 31.4 pounds, but (because aluminum is less dense than steel) it's around 4 pounds more buoyant than an HP100 at the end of a dive. Since you need to make up that difference with lead, the net effect above water is that a rig with an AL80 is 1 pound heavier than the LP85 or equal to the HP100 with its extra pound of gas.
The HP80 is the way to go if you really need to save weight. It's around 7 pounds lighter than a full HP100 and 1 pound more negatively buoyant, for a net 8 pound drop in rig weight. The biggest issue with the HP80 is that you need at a true high pressure fill to match the gas capacity of an AL80*. So check with the shop you will be using most of the time to see if you can get a true HP fill.
*An AL80 actually only holds around 77.4 cubic feet of gas at its standard rating of 3000psi. The HP80 holds 80.1 cubic feet at its rating of 3442psi. If you have a shop that only fills to 3000psi, your HP80 will have around 70 cubic feet.
