I would think for that type of gun it really wouldn't be much of a difference. Although a HPA tank may give you more shots per fill and better consistency since it has a regulator.
actually there is a bit of a difference... HPA works a ton better. the flow rate on this airgun is a little higher than a ihav.net gun, and the HPA can supply the pressure more consistently at that flow rate when compared to CO2.
what is all this about needing a bulk mag? when you buy a full auto MP5 does it come with a drum mag? what about when you buy a full auto ak-47... does it come with a 100 round clip? no- they are aftermarket upgrades; just as the bulk mag for the GSMG.