There's no contest. The only reason to run CO2 is the reasons stated above - availability. But, if you can get, get it.

I remember the last time I used CO2, it was the only time. It was about 40* out, I was at the chrono w/ my newly purchased A-5. I got the velocity adjusted to 280 - then proceeded to take a few shots in succession. First shot 280, then 250, 220, 205, 190. All of five shots less than five seconds to dip below 200! That sucked, but I played that day as is. Bought my HPA tank that week, a used 72ci/3k steely. I haven't looked back. I may not get 1000 shots like I would have w/ the CO2, but I'd rather have 650 +/- consistent ones.

EDIT: Weight issue as asked above - It depends. A full 20 oz. CO2 tank can be heavy because the CO2 is in liquid form (well most of it). Whereas an HPA tank is just full of air, weighing only as much as the tank itself. A steel HPA tank like I have is a little lighter than a full 20 oz. but not a whole lot. A carbon fiber tank on the other hand if pretty light compared to a full 20 oz. Weight doesn't matter to me though, I run remote.