There's two sides to every coin.

CO2 is a LOT faster to use. It also costs more and, if you run out or cartridges, you're also out of luck.
Pumps are more eco friendly and you never run out of air. A guy my age, however, might not have enough time left to use one of those teensy mini's. Everybody says how fast and easy their mini pumps get up to 120 psi. Hey - I'm from Missouri - don't tell me, show me.

So how often do you have a flat? If it's not too often it doesn't really matter. Just suck it up and use whatever you happen to have with you. On the other hand, after having a flat you're probably already unhappy. If that's the case, CO2 is better because it's so much faster - unless, of course, you screw it up and let all of your CO2 escape.

I carry CO2 on my tandem and a mini-pump on my single bike. On my fixed gear I don't carry anything with me but a positive attitude.