From a crewing perspective there are far more Challenger pilots in Europe than GIV pilots, largely because the type rating is common with a type still in production. Most GIV owners have moved on to something a bit more up to date.
Having said that, if you want good crew you are far better off finding good people and type rating them if needed,rather than hiring idiots who happen to have the right piece of paper. It can cost you a lot more in the long run!
Good luck.