My two cents:
G90 Pros and Cons:
+ Better beam control (all controls electronically, including CPC magnets)
+ Smaller spot size (uses LUGs by default)
+ Better stock lenses (HD10F)
+ Better geometry allows for side-by-side stacking
- More noise (can be modded)
- Green color is not accurate. Change C-element.
- More difficult to repair/find spares/mod
- No mods readily available
9500 Ultra Pros and Cons:
+ Better video chain (especially 03P). This can be modded even further.
+ Very easy to repair/find spares
+ Mods readily available (Tim, MP, etc.)
+ Can be upgraded with better parts (HD10F lenses, LUGs, frankenyokes, etc.)
- Worse geometry controls. I believe side-by-side stacking with Ultras is NOT feasible
- Worse beam control (I think the default focus yokes are the worst of all 9" machines, and start blooming at contrast 60ish).
- Worse stock optics for stacking/blending applications (GT17/HD10 vs. HD10F)
- Worse stock tubes for stacking (you'd want a smaller spot size for stacking)
- Red color is not accurate. Change C-element.
I have never seen a very conclusive answer on the inability of the Marquees of side-by-side stacking, but the Marquee manuals always refer to top-bottom stacking and this image (from the Reference Imaging website) is very telling:
IMHO, the G90 is a machine you'd want not to open or tweak, and cross your finders it doesn't break; whereas the Marquee is a tweaker's dream
Regards - Andres