Definitely AEG for a first gun. GBBR's are nice, and more realistic behaving then an AEG. But they're more complicated to care for, and very pricey, not only the gun but the mags as well. Run an AEG for at least a year, learn how everything works, then maybe you can consider changing to a GBBR.
Personally, considering the temperature issues plus mag cost plus increased maintenance requirements, I don't think I'll ever get one. My AEG is realistic enough for me, works in any reasonable temperature and the mags are dirt cheap.