Seeing how well the lamp is holding out on my X35 I would take the X75 too if it was up to me. I've barely lost any output after 150 hours so I'm still able to run at -15 aperture for 16:9 content and still achieve 15-16fL The extra contrast of the X90 could be lost if the iris has to be opened up more than the X75 in the same installation, plus the lamp is slightly brighter to start with anyway IIUIC.
I'll be interested to hear if you stick with using the lens: I debated selling my Isco II and getting the X55 rather than the X35 plus my lens (I already have a Lumagen for CMS and a Darbee), but the three (zoomed) X55s I've seen don't seem as sharp as my long throw X35 plus lens.
"Don't believe everything you read on the internet". William Shakespeare 1615