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I use doubles because I'm always in cave gear even in open water. I don't feel it's fair to not pay double to fill my tanks (32% Nitrox) because I get two or three dives out of my doubles.
I like buying gas by the cubic foot. My buddy has better air consumption than I do and two single HP100s. To match his air, I use twin 72s. I don't mind paying a little gas guzzler tax, but paying twice as much for 130cf in twin 72s as it would cost for a single HP130?