Hi there.
I would definetly go to with the K2500 for many reasons. Firstly, its double the polyphony, K2000 has 24, but the K2500 has 48. You also have many other upgrade options such as KDFX(a multiple digital effects processor which allows you to assign different effects to different tracks. The upgrades such as P-RAM,Sample RAM allow you to expand much more. The K2000 only allows you to upgrade to 64megs of sample RAM and 760KB of P-RAM(or sequencer memory). The K2500 has that almost doubled. Also you have options with the K2500 such as Live Mode. The K2500 also has multiple data sliders which you can asign multiple effects.
Both the K2000 and the 2500 are still great synths and are still very much supported by
its company. But you have so much more with
the K2500. I realized this myself after being
a K2000 user for about 2years. I picked up a K2500 myself and have been a happy camper so far. There are so many other advantages, take a look at Young Changs website for more details. Good luck on your decision!