That's always the argument between pistols. I personally like ones that run off green gas because it's more readily available at most games in the form of propane tanks with propane adapters if you happen to run out or forget it at home...
I believe that it also is a better value for your money. Then again, if you don't use your pistol that much or if it's non-blowback Co2 can also last a long time.
For the most part though, many of the high end companies that make more accurate guns run on green gas. I think when it comes to blowback, Co2 just tears shit up too much! :-)