I did a lot of research a few years ago comparing the Falcon 900EX versus the G-IVSP. We (the boss) decided to purchase the 900EX. We flew both aircraft from Aspen-Oklahoma City Wiley Post Airport-London Luton and return via the same routing except for stopping for customs at Bangor Maine.
Bottom line for choosing the 900EX was that it outperforms the G-IVSP out of Aspen where the boss lived.
Other contributing factors were;
1. Foward galley. I know you can have a foward galley in the G-IV, but it will cost more.
2. Three engines. As we were planing considerable Pacific operations the boss wanted three engines, and as an old 72 driver I couldn't argue that point.
3. Wider passenger cabin.
I flew both aircraft and to be honest the Falcon flies a lot nicer than the G-IV does. However, this was not a contributing factor, the boss didn't care which aircraft flew nicer.
Now on maximum long range missions the G-IV is faster than the 900EX, but you pay for that with higher operating cost.
Do not get me wrong here, the G-IVSP is a hell of an airplane, no doubt. I would have been very happy to have bought and operated the Gulfstream.
But, the bottom line is that the Falcon 900EX just does a better job regarding 'high and hot' airports than the G-IVSP.
One more point, this was before the EAsy Cockpit became standard on the 900.
Good luck, you'll enjoy flying either aircraft.
Please feel free to PM me if you have any questions.