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Green Gas is propane (usually with some silicone lubricant pre-added and a different odorant).
Co2 is Co2 and also rarely also referred to as Black Gas.
Propane is cheaper (you can use those Coleman camping tanks) and is the defecto gas used in gas guns.
Co2 is usually in the form of those 12gr disposable cartridges and will cost more to operate: it does provide more power in a lot of guns (which can be a good or a bad thing, as sometimes it will push muzzle velocity over field limits) and will function in slightly colder weather.
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