I like propane, because it's dirt cheap, and I only have to use as much as I want.
If I want to go target shoot 5 rounds in by backyard, easy. If I want to go shoot 100, just gotta bring the can.
My old P99 used C02. Was nice, because one little cartridge lasted an entire day. Past that, meh, but that was gun design issue, not propellant. On a good note, it doesn't smell like really anything.
Wouldn't go back to CO2 myself. Disposing of the cartridges, eh. Also, CO2 was kind of destroying my gun. Too much force, on a gun designed for CO2. The frame was cracking, which is NOT very great. Also, at least for playing games, CO2 tends to push FPS limits.
Propane's downsides, filling midgame is a bit of a pain, and you only get so many rounds a mag before you gotta gas up. It smells terrible. You will need to lube it yourself, lest your mags start leaking.
I'd buy more based off of the gun than the propellant. If both are an option, propane is my pick. Good luck with whatever YOU choose.
EDIT: Also, be aware, there are some cheap CO2 pistols that... well they're cheap. CO2 guns have a higher chance of being low-grade (Not to say you can't get a decent one)