I always have my hydration pack with me so storage isn't a problem. I've been leery of the CO2 cartridges since I've had to loan my pump to more than a few riders who had cartridges that failed on them...
The BF has only ever had one cartridge fail on him, and I've had only one occasion to use one, so I'm not a useful sample. I carry both cartridges and a pump, because tires have a tendency to jump off the rim sometimes...