Propane vs Top gas!?
Doesn't anybody search anymore on this site? There is a thread about this that just opened up. Sorry if that sounds blunt.
THere is also a link on that thread that compares propane and gg.
Anyways, the good thing is that if its a WE M9, you can swap Marui M9 mags on it. They are much more reliable than WE's mags, IMO.
If I remember correctly, the WE M9 have metal slides which should propel you to use green gas/propane for a smooth cycle. CO2 tend to wear out some parts quicker than propane because its pressurize higher than propane.