The difference between AL and Steel is great. The steel tank distributes the extra weight nicely over the length of the tank, instead of having the extra weight in one spot. Going from AL80 to HP100 I was able to remove 8 lbs of lead from my weight pockets. You get much better weight stability and trim when the weight is spread over your body. Also the steel tank is a little smaller.
Adam
I confirm exactly the -8lbs when moving from AL80 to HP100, which is very very nice. I would also say that the steel tank is really smaller than the AL80 (look more than an AL72).