Here's my take on GLEX vs. Gulfstream:
We looked at G450 and Global5000, among others. Coming from a CL 604 the boss' first impression of the G450 was the too-narrow cabin. We wanted a forward galley which Gulfstream was not willing to do. This was the deal breaker.
We now have 900+ hours on a new Global5000 and apart from a few teething problems the plane has been very dependable. Cabin systems have been the biggest issue/disappointment for Global owners. The new cabin Collins CES (Cabin Entertainment System) is reliable.
Only gripe: Global relies heavily on computers so many BITE tests as things are booted up. She requires a gentle touch and a patient pilot until things are up and running properly. After that...all good.