GBBRs are usually less likely to break down than AEGs, it's the mags that usually develop leaks.
AEG's are all TEMPERATURE, GBBRs can't short out in the rain (HPA can be all temps)
GBBRs tend to have better stock range and accuracy, have fewer upgrades & therefore cost way less to upgrade fully, and typically have REALLY good performance in their temp range.
Ease of maintenance, need a gundoc to do anything cool to an AEG, anybody can work on a GBBR, there's almost nothing you can screw up.
AEG's always shoot around the same FPS, GBBRs change muzzle energy based on temperature, weight of ammo, maintenance (dirty bolts reduce fps), and gas used.
GBBR's will perform better with heavier ammo vs an AEG shooting the same relative velocity, since the inital pressure is much higher and allows you to impart more hop to the BB without losing drastic velocity.
EX: an AEG shoots 330fps without hop with a .3, you add hop and now it shoots 270
a GBBR shoots 330fps without hop on a .3, you add hop and the fps may very well increase!
Now you could increase the spring in the AEG to maintain the fps after hop, but that would most likely breach the field limit since most people test guns with the hop off.
AEG mags are dirt cheap, GBBR mags are probably going to cost you as much as or more than the gun.
trigger response....well AEG's have potential to be just as fast, but GBBRs are typically quicker.
Good all season workhorse vs realistic awesomeness
Pro's and cons to both.