Co2 bottle are usually constructed of steel....they are only designed to store Co2...@900 psi or so
Compressed air botttles are generally made from a coposite Kevlar wrap around a steel or ally bottle.
They are designed to store air at up to 3000 psi or 4500 psi or 5000psi....so as you can see, putting air into a Co2 bottle is a bad idea....
However you can put Co2 in an AIR bottle.
Air is better than Co2 if you can get it, its more consistent (which aids accuracy) cleaner, and doesnt make your marker freeze up...however it is stored at ridiculous preassures and needs to be treated with a great deal more respect than co2, which demands enough respect as it is!
Whichever you choose, just remember that that is a bomb you have nder your chin..its just a question of how powerful that bomb is....