Definitely rubber dome. You have to be careful with the SGI boards too, if you're planning to use it with a PC. Only the 950900 (granite) and 9500829 (beige) are PS/2 compatible.
http://deskthority.net/wiki/SGI_GraniteAlso, if you do manage to score one of the PS/2 versions, it probably won't work with the typically recommended "cube" USB adapter (which works fine on a Model M). Instead, there is a Belkin adapter that works great.
http://www.amazon.com/Belkin-USB-PS-2-Adapter/dp/B000ELSXFYI'm using it right now with my new 950900 to type this.

I like these damped ALPS sliders, but it feels kind of weird. I need to give my fingers some time to re-adjust to ALPS typing. The key travel is noticeably shorter than the Cherry switches and buckling springs.