Get Both
Got two flats on a winter ride years ago. Had one CO2 cartridge and no pump. Got home and bought a pump right away. I use the Lezyne Road Drive; I think the Pocket Drive might be better suited to a mountain bike tire. Road Drive mounts to the side of my water bottle cage. I've used it lots of time, either for my own flat (get the tube started or top off after CO2) or for a friend's.