@Marcus
I can absolutely /sign your experiences - unfortunately i´m no owner but a pilot, and i only had the pleasure to join a "friend" of mine on his demo flights on both G550 and GLEX.
First of all i can say that i adore the Gulfstream interior design - i really was disappointed by the GLEX cabin. No real chance to compare it, especially the natural light is far better inside the Gulfstream thanks to the beautiful windows.
Second i prefer the Gulfstream avionics - in my opinion they seems to work more effective and i felt much faster quite comfortable in the cockpit.
Also the Gulf demo team seemed far more motivated to sell an aircraft, but maybe the Bombardier guy simply had a really bad day.
After all, if i get the chance to fly on one of both aircrafts in the future or if i should suggest the "better" one, i would choose the Gulf, although i also really respect the GLEX as a great aircraft...