PerroneFord:Air tanks and nitrox tanks are the same. There is no difference.
Keep in mind that the nitrox cylinders will need to be O2 clean and to keep them that way you will need to make sure that any plain air fills are with suitable air to maintain the O2 clean condition, not all air fills are.
I think that's the crux of the variable use cylinder. After that it's a matter of what sort of diving you intend to do. Generally if you are actively diving you can't have too many cylinders.
Pete