They're all nice airyplanes, but the G450/550/650 is on its way out, and there will definitely be a full new type-rating course to be done to transition to G500/600...
Global pilots have taken in the highest freelance rates/salaries in that segment over the last few years (as compared to GLF and Falcons..), but that is not bound to stay so..
I would personally go for an 8X rating, which includes the 7X, and places you into easier reach of 900 Easy and 2000 Easy ratings... Then again there has just been some pretty bad news for Falcon Jet with a renewed delay to the 5X program...
After 26 years in the business, I have definitely stopped caring about the hardware, and only salary/free-time considerations apply, second only to not working with muppets...