Right now I'm using AL80s for doubles, since steel tanks give me no need for additional ballast (even in my drysuit) which I don't like because I'm new to doubles and their head-heaviness. I just attach 4 lbs of lead via a camband to the tail of the tanks and trim is much easier for me.
My first set of doubles were HP100s that I have now broke down to singles and they were great tanks: not too heavy, lots of gas, and trimmed well for me. I just didn't like the fact that I had no ballast that I could move around to make them trim easier. Also, the 80s do just fine for most of my doubles diving that I do.
For myself I worry about gas capacity very little when selecting new tanks (says a guy who owns Fabler LP85s, LP95s, PST HP100s, an Asahi HP120, and Catalina 80s) and instead focus on how comfortable they are for me. Lately I've been diving the LP85s and loving them, nice and light on land but for the majority of my diving they have enough gas for me, trim out very nicely, and the bouyancy characteristics are favorable for me.
Peace,
Greg