i use a CO2 inflator with two cartridges because it can inflate a tire to around 120psi in mere seconds. i don't carry a back-up pump because if i have more than 2 flats on a commute (it's never happened, knock on wood), i can simply hop on the el or hail a cab and throw my bike in the trunk to get to work on time. i live in a city with a very comprenhensive public transit system; the idea that i would ever have to walk home with my bike because i was out of air is absurd.