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There is no good reason to buy an AL100. It weighs way more than an AL80. It's bigger in diameter. So, the added gas you gain by moving to an AL100 is diminished by the added diameter of the tank (more drag through the water) and the weight of the tank. It's a beast. An HP100 would be a better choice, albeit more expensive.
The difference should be able to figure out with pretty basic math. 100 cubic foot tanks filled to their capacity generally have 100 cubic feet in them. 80 cubic foot tanks generally filled to standard capacity have 77.4 cubic feet in them. (Off the top of my head, I am not sure if the AL 100 tank has less than its named capacity the way an 80 cubic foot tank does.) That amounts to nearly a 30 percent increase, so you should expect that much additional dive time.
20/77.4 = 25.8 percent.