We purchased a 900EX after a great amount of research between the G-IVSP and the 900EX. We conducted a fly-offs between the two aircraft on trips that fit our trip profile. The fly-off were to see which aircraft could perform the following legs non-stop.
Oklahoma City-London/London-Oklahoma City. (Both aircraft could do this leg.)
Aspen-London. (Only the 900EX could do this leg and only when the OAT was -05 c. with good weather at Aspen. The G-IVSP could not do this leg no matter how cold the OAT was in Aspen.)
Aspen-Bogotá Colombia. (Both aircraft could do this leg, however, in the summer when the OAT was above, I believe 15c, the G-IV was unable to go non-stop.)
Plus a few others that both aircarft could do non-stop.
As the owner lived for the most part in Aspen and secondly in London, the non-stop Aspen-London leg was the most important to him. The only other two aircraft at the time that could go Aspen-London non-stop were the Global Express and the G-V, both of which were out of our price range. Therefore we chose the 900EX.
As for cabin size comparison it may interest you to know that the 900 has more space in the 'living' or passenger cabin area than the G-IVSP. This area excludes the galley and the aft lav. A small point and a small amount of square footage I'll admit. The size of the galley in the 900EX never caused us any problems.
One more point that was important to me and the other pilots was that loading baggage into the baggage compartment was a hell of a lot easier on our backs with the 900 verus the G-IV. A small thing admittedly and not something the passengers don't really care about, but was never the less a factor.
Other point, going across the Pacific Ocean having three engines, compared to the two, was a nice comfort factor to me at least.
Another point is that 900 was much lighter on the controls that the G-IV. The G-IV reminded me more of a 727, where the 900 flew more like a Sabre 40 or a Lockheed Jet Star.
However, in summation, I would have been just happy flying the G-IVSP if that been the aircraft the boss chose to buy. Both aircraft are great and again I would been happy flying either one.