Either way you decide to go, just make sure the first time you try to use it isn't on some dark street. I have a CO2 inflator that's in the seat bag of my wife's bike. I'm having second thoughts about that since I'm not sure she'd remember how to use it if she needed to.

I carry a Topeak Road Morph and keep it attached to the frame. It takes up no room in my seat bag that way. Personally I'm better off with the pump. It's a good one and take no more than five minutes to pump up a tire. Not much more work than a floor pump. Because they'd end up in the seat bag, cartridges + inflator would take up more space. Plus this way I don't need to worry about remembering to replace a used cartridge.