Propane is pretty much the staple of any gas blowback to date. C02 if anything is a novelty/convenience to it. But it does have its benefits.
Usually C02 will shoot harder, and will be a bit more reliable than propane in colder conditions. If you live in an area where the temperature is generally lower. You should opt for C02.
Propane however is a LOT cheaper and reliable enough. 80% of pistols and rifles set for 'gas' run off of it. It just starts seeing some performance issues in colder or hotter temps. Really depends...