I've been having a debate with my dive buddy for an upcoming trip. His S.A.C. Is larger than mine and he is typically down to 500 PSI while I am around 1250 on an aluminum 80. Most of our diving take place between 60 and 100 feet. When we dive air, nitrogen is almost always the limiting factor for me whereas it seems that air remaining is always the limiting factor for him irregardless of the FiO2. We have choice of either 100 cubic foot air tanks or 80 cubic foot nitrox tanks. Any suggestions on which will extend the bottom time for someone who goes through air fairly fast?