I went with a gas gun first, and all that came afterwards were GBB.
Love them, and love them, what's there to say? Yes it's a lot of work and maintenance but I find it easier to work on then AEG's.
Huge factor depends on your budget, so.. if you have a low budget I would go AEG. They are also tend to be more reliable, and run well if you play winter outdoors.
Example of a price gap would be... 1 gas magazine can run you $30-50, where 5 aeg magazines can run you for the same price.