Just remember:
Smokeless powder doesn't need to pull O2 from the air.
...but YOU do.
If you go the CO2 route, be aware of the air quality in the room, should you ever need to fight the fire. Oxygen-displacing extinguishers (CO2, Halon, Novec, etc) only worsen the conditions created by fires in the home/garage/shed/workshop/etc. If you pass out from asphyxiation, you probably won't have any luck putting the fire out.
