Depends on maximum storage pressure for the same size of storagebottle.
If its 1000 psi CO2 wins hands down. If it goes over 4500 psi, air wins.
CO2 goes liquid at room temperature and increases density with a factor 10= compared to the compressed CO2 gas on that pressure.

Imagine a 3L bottle. Rules say you can fill this bottle with 2 kg of CO2, or 3L of compressed air on 1000 psi/70 bar. With air you have 3liter*7a bar =210L of gas.
With CO2 you have 2 kg=512L gas per kilo, so 1024L gas in that same bottle.

To beat this with compressed air youll have to go over 337 bar.

600grams CO2 bottles, 20oz paintball bottle with On Off valve can be bought over here around ‚45.