I also carry both, a pump and several cartridges. The CO2 is definitely much faster than a pump, but for the most part it's a one shot deal.( I know there are adaptors that will stop the flow of the CO2 once it's punctured) I like to give the tube some shape before installing it ,and then I finish it off with a cartridge. I carry both simply because I have seen several instances where someone has had multiple flats. I have also seen where the CO2 cartidge isn't any good, although that's a rare oddity.

For me personally, I would rather carry the little bit of extra weight of both a pump and cartridges, then to have to walk my bike home.