The Kawai CE220 is actually a step up from the KDP90 in three categories: sound technology, key action, and price. More at http://bestrateddigitalpiano.com.
The CE220 features Progressive Harmonic Imaging, which (according to the manufacturer) “uses a larger tonal database than regular Harmonic Imaging to provide greater tonal details throughout the dynamic range.”
To me, that essentially means they used more of the data they captured during the sampling process in this piano than in the KDP90.
This piano also incorporates a different type of hammer action called Responsive Hammer action, in which they’ve added a simulation of that sort of “notch” feeling you get when you play softly on an acoustic grand. More at http://bestrateddigitalpiano.com/how-to-get-the-best-upright-digital-piano/.
Plus the keytops on the CE220 are made to feel like ivory. That said, you’ll also pay several hundred dollars more for the CE220 than the KDP90, so you’ll have to decide for yourself what’s more important.