Hi Simon,
Well, the best place to look is at Kurzweil's site. I don't know that the sounds are any different. A big difference, for me anyway, is that the K2500 has 48 notes of polyphony while the K2K has only 24 notes of polyphony. I believe that the K2K cannot be loaded up with as much sample RAM (128 MB on the K2500) and I think that it has less PRAM which is where programs, sequences, etc. all get stored. In terms of ROM sounds, I believe they come with the same set of ROM waveforms so, if that is all you are after, then go for the K2K. If polyphony matters to you, go for the K2500.
There's probably a few things I've missed as well which should be covered on their website. Myself, I'm currently looking at the K2600 but for other reasons.
HTH,
Fernando