If you want to maximize your bottom time then you should really get double tanks. The common solution is to have two large tanks mountend on a metal back plate on your back. You dont need to be a technical diver for that. Double tanks simply gives you double the gas. You just need to watch your depth to avoid deco. A less common alternative to back mounted tanks is to sidemount them, but that's a whole different story.
If you dive in a drysuit then you will want steel tanks and steel back plate and no lead. Else you dive wet with aluminum tanks. It is generally better to have physically large low pressure tanks than small high pressure tanks because large tanks float more and only replace your lead without making you way too heavy.
Here in Europe the Eurocylinder (ECS) 12 Litre 230 bar double tanks are very popular. That setup gives you 24 litres of gas at around 210-220 atmospheres. It's almost 5400 litres or 186 cuft realistically. The larger 15L cylinders are rarely seen, and those who must (and their friends), carry 18 L 230 bar doubles. Those are not fun on the shore. Single tanks and drysuit usually means 10 litre 300 bar cylinders (because of the weight), but some people opt for 230 bar instead and larger physical size (plus carry lead). I do not know US tank sizes, but I am sure you can find something similar as the sizes and weights have usually been calculated optimally.
