AEGs are proven to work in any weather, they require very little maintenance, they are ultimately cheaper to operate. Beside if spray and pray is your concern, buy a bunch of low-capacity magazines (or real-cap if you go for a more common model). AEGs also have a much greater diversity in terms of brands and models to choose from.
GBBR are relatively new, they require more maintenance, the magazine are expensive, cold weather affects performance significantly. There is also a somewhat limited choice of models on the market.
Your better off starting with a good AEG (you should research and read - before making your first purchase since it will break or make your airsoft experience)