I carry both. A mini-pump attached to the water bottle mounts and the CO2 in my seat bag. In really cold weather (i.e. below freezing) I've found using a pump difficult with hands that are getting stiff after changing out or patching the tube. In those situations that CO2 is a welcome relief. Otherwise, I use the pump. (OK, once when I was going to be late for an important work meeting I used the CO2 because it was faster.)
__________________
A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831