I may be wrong but don't the 2000 and FP90 use the same keybed and Piano sound source ? The 2000 adds a second sound source for all the extra PCM sounds. And has more controller capabilities.
Keybed same
10 instruments with V-piano limitless polyphony vs 6 super natural piano modeling sounds (limited edditability) on the fp 90
1100 vs 350 super natural sounds, 128 poly on the 2000 and 384 poly on the fp90
Effects engine of the RD is much more advanced thne the fp90
Very limited controll on the fp 90 vs advanced master keyboard functions on the RD
RD has audio over USB, which effectively turns the RD in an audio interface for a connected computef...
Advanced midi on the RD2000, 2 midi outs, rainlink to PC
Except for the keybed, i dont think there is a lot the same on these 2 instruments..