Very much personal preference- some people around here will carry a lot of spares/tools around just in case, others are ok going without and accepting the risk that needing a lot of that stuff is low. I average a little under one flat per year over the last decade, so the convenience and packaging of CO2 works well for me, and has been reliable. For someone that flats much more frequently or goes on longer rides, a pump and more spares/redundancy make sense.

Whichever option you go with, if you aren't already comfortable with it, practice getting a flat at home so that you aren't learning on the side of the road, or realizing you don't have all the tools you need.