I agree with Uncle Dave, about the (only) slight improvement of the "overall effect".
I also didn't find there was a considerable improvement in many of the sounds. Many styles are new, but I find the (excellent) overall quality in the X1 not inferior to new keyboard.
In my opinion the acoustic piano sound is not a reason in itself to change keyboards, as there are optional soundbanks for X1, containing much better piano samples - that you can load on startup. I'm always using these new samples, that I find close in quality to the ones offered in the XD9.
I would also add the advantages of some new features introduced. Drum mix, bassist, rootless chord recognition (great for new live performance horizons), morphing, and a practical possibility of user pattern rearrangement in the memory.
-- José.