The Casio PX850 is another instrument comparable to the KDP90.
It has 256-note polyphony, and 18 built-in tones. The keys are made to feel like real ebony and ivory, and it comes in black, white, or brown finishes. More at http://bestrateddigitalpiano.com.
Like the KDP90, you can split the keyboard into two identical zones for teaching purposes.
But unlike the KDP90, this Casio model offers USB connectivity directly to your computer, and, with Apple’s Camera Connection Kit for iPad, you can actually link those two devices. More at http://bestrateddigitalpiano.com/the-best-yamaha-digital-piano-that-you-can-purchase-today/.
Another cool feature of the PX850 is lid simulation, meaning you can set it up to sound like a grand piano with the lid either open, closed, half open, or removed.