aeg. gbb may be more realistic but less efficient price wise, greater wear out speed more maintainace, and more likely to just not work due to cold etc. deff go aeg, specially for first few rifles...
I love my GBBR it is an awesome gun to shoot, but generally only use it half the day max then switch to my AEG when I'm fed up with carrying 10+ gas mags around LolZ
Personally I prefer GBBs to AEGs due to how much more fun they are to shoot (weight and recoil makes it feel more "real") but I run AEGs since I don't want to carry a lot of heavy mags and it's more consistent/practical.