I carry both. For my commuter bike, I usually replace the tube, and patch the old one. That way, I have a spare for the ride home. Also, there are times when I can't find what caused the hole. So, I hope it was just one of those in and out type of punctures. But there have been times when the cause was there, but just under the casing, so later another hole in a tube. With a patch kit, I can repair this and go on.

I use CO2 for my go fast bikes and my fixed gear. For my daily commuter and tourer bikes, I use a frame pump. I have never had a problem with the CO2. There is one problem though, as with my answer above, if you repair a tube but didn't get the cause fixed, you could end up with another flat. Unless you have more than one cartridge, you are out of luck.

I like how little room they take up. I can put it in my saddle bag. I don't need to put it on the frame, so more room for water bottles.