Additionally, even if 2 keyboards use the same 'chip', their onboard sound libraries (as Paul said, the sound engine) are different (with Yamaha anyway... interestingly with Korg, the PA700, PA1000 and PA4x have the exact same preloaded sounds, but with very different sample ram and hardware configurations).
Is the SX900 and Genos rom (preloaded) sound library the same? No. Are the A5000 and SX900 the same? No; in fact having all of them side by side, the sx900 is more or less a Tyros 5 with all the SA2 sound stripped out, and a Live! version of the Revo drums on Genos thrown in for compatiblity, basically an entire hardware generation behind Genos (which is what they always do). The A5000 is unusual; all the previous Oriental arrangers were based on the mid level e.g PSRA3000 based on s770, etc but the A5000 actually uses the PSRsx600 sound library, but adds a *ton* of Middle Eastern specific sounds (insanely detailed Oud's and Arabic Drum kits).
Is there anything the Genos can do that they can't? the sx900 doesn't have Kino strings (particularly noticeable are the spiccato strings), doesn't have the resonantor guitars, doesn't have Revo drums, doesn't have the same detail in the piano. But on the other hand the sx900 and A5000 have built in speakers, which the Genos does not. The Genos can do multitrack ping pong recording (you can keep overdubbing with a new live layer while playing back a previously recorded layer) or you can multitrack 2 stereo passes for a single song (all of the PSR's can only do a single take, although the sx600 includes an audio interface). The Genos has SA2 voices for phrasing, the PSR's don't. In fact, *none* of the Sounds that the Genos introduced over the T5 are on the sx900, except the paired down live versions of the revo drums. In fact the sx900 and A5000 both have a *lot* more drum kit options than the T5, the latter even having exclusive oriental Revo kits.
The Genos (and T5) can also do ensemble mode, which is great for some specific applications.
Chekc out its use in my Raiders Theme video at 2:06 until 2:20, when I switch back to normal R1+R2+R3 playing.
https://www.youtube.com/watch?v=4GqrFFM_I0gThe same section of my Raiders registration on boards that don't have ensemble mode sounds clunkier, and a lot less delicate.
That's not even including the 3GB of available sample area (actually 3072 mb) versus just under 1GB for the sx900 and A5000.
And yes, as Paul pointed out, don't discount the 32bit digital audio converter on the Genos vs the 24 bit ones on the PSR's.