Both. I got a Bontrager Air Rush Road

Used to be a CO2 hater. Then I used one once when I had a flat. Wow. Sure beats working a mini-pump for hundreds of strokes. I carry a cartridge with me, but the Air Rush gives me a pump backup, too. For not much more space than the pump alone. Also carry one spare tube and a patch kit.

CO2 gives me a better chance of being on time if I get a flat on the way in; better shot at getting home quicker on the way back home. Steel cartridges are recyclable.