I carry both. CO2 is much faster and easier to perform. However, you get one shot at it (well, two because I carry a spare cartridge). The pump is handy for recurring problems.
Cartridges are not a big deal - 15, 12 oz for $8 or $9 at Wal-Mart or other sporting goods stores. Handy suggestion for those running high pressure is to pump some air in and then use CO2 to get to pressure. Haven't tried it (or needed to) but sounds like good advice.