Gas - as a quick reliable answer if you wish to use it skirmishing
GG usually hits 300-320fps aprox
CO2 - often hits over 500fps on a fresh bulb settles at about 400fps until it starts to deplete so too hot for skirmishing
Green Gas - if you use a lot you can use propane/Colemans (no not the mustard) with a tiny tiny squirt of silicone
very cheap in long run but a little smelly, you can buy the kits
Some real cheap nasty co2 bulbs don't seal too great and almost bust a cheap plastic mag trying to tighten it further
So yeah this is why most run Green Gas over CO2
there are restrict/adapter/mods to lose a little power from co2 guns, plus you can buy a refillable co2 bulb you fill with green gas
but in a nutshell - go for green gas option to save hassle