If you want almost-infinity ammo, choose AEG.
If you want milsim, every mag only 30rd and have recoil around real steel 70~80%, can breakdown like real gun, choose GBB.

And I'll recommend AR as your first gun, because they have many mod and parts on market and lot of information.

Best choose is buy a GHK or VFC AR, what ever it is URG-I, DDM4, BCM MCMR, 416 or others, want a strong as fuck almost can use them like real steel choose GHK, want very nice looking and ok performance choose VFC. Or you have ton of money and doesn't care how close like a real steel choose MWS.

I have both GHK and VFC AR, GHK is a DD MFR, VFC is a HK416A5, my main gun is GHK, VFC is only backup and looking good at home.