I would have to agree with Morph that Yamaha makes really good piano sounds. This coming from a Roland user.
I guess it depends on your style of music. Korg is usually better at electric, synthey pads, and maybe some more far out space kind of sounds. Roland is usually better at real instrument sounds. I don't know enough about Yamaha to comment intelligently.

Morph, does the motif rack have the sequencer and sampler in it? I've already got a sequencer; I've just been looking to expand my sound collection. i'm a producer as well as a performer, and I'm starting to wish I had some new wave forms.