For some reason, I carry both CO2 and a pump. I had gotten in the habit of using CO2 cartridges a while ago because they can fill a tire so quickly. However, I got several flats during my commute one day, ran out of CO2 cartridges and had to walk 3.5 miles in my cycling shoes. I have carried a pump ever since then, but continue to also use CO2 for its convenience.

CO2 will leak out of tires much faster than regular air. It get you home or to the end of your ride just fine, but your tire will be spongy the next morning.

BTW, if you buy a CO2 inflator that doesn't require threaded cartridges, you can save a lot of money. That's because you can buy un-threaded cartridges in boxes of 12-24 at bigbox or hardware stores for much less money than threaded ones.

If you decide to get a pump, the Topeak Road-Morph with the gauge works great, as well as their frame-fit pump.