A friend that went from a Global to the G650 says check landing perf if you will use short runways. The GX, from his experience, will comfortably use runways that are a bit dicey in the G6. Also, you can't use KASE, yet.
His crews are skeptical right now. It will burn 4,000# less fuel on an Atlantic crosssing than a G8, so there's that.
GF