A well tuned AEG will perform better than a well tuned GBBR.
AEG's will have a longer enagagement range (accuracy on target at range), and more consistent follow up shots.
GBBR's do not perform well at all below 10'C. Cool down rapidly affects both cyclic rate of fire, and gas consumption. Mags will often fail and "dump" their contents in colder weather.
GBBR's have benefits of operating closer to the real thing, they have recoil, and have near instantaneous firing (no need for the AEG wind up of the piston to fire).
In short, AEG's fire faster, shoot further, and are more accurate.
GBBRs are excellent training tools, have much faster engagement speeds, and are far more fun to shoot. But they lack a lot when it comes to actually hitting anything outside of 150 feet accurately.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.