Hey Brian,
So I've always appreciated the fairly simple Epson naming conventions, but replacing the 9880 with a 9890 and then trying to discuss the difference between that and the 9900 is a bit obnoxious. Reminds me of the days when we had the 1d and d1.
Any way, the 9890 is basically the 9900 engine minus the O & G. They both work with the Spectroproofer under EFI, although you would need to wait for the next XF version for 9890 support (should be this month). If you are just printing 4c and not using spots then the 9890 is a good choice, although I would suggest holding out for the 9890 over the 9880 as the 9 series is just much much faster, the cutter is better, and paper handling is easier. We've installed tons of the Epson's and the EFI package and it is a pretty slick combination white the spectroproofer. What version of the RIP are you running? I would suggest that you upgrade if you're not on 4.0 or newer as the stability when using the spectroproofer has gotten much better.