this is my generalization based on yamaha, korg, and roland use in the past, i dont know it might be different to each person but anyway.
korg sounds generaly seem more natural/realistic when its acoustic you are looking for, such as grand pianos, violins, acoustic bass, etc. the classic korg M1 is a great example. maybe best for a funky acoustic natural groove approach.
roland has a more synthetic sharp sounds, esp drums, synth strings, artificial atmospherics. these are great for house, and freestyle, esp of the 80s variety if you want that robotic, techno/electro sound.
yamaha maybe in between, i dont know as much about latest yamaha developments these days. though yamaha keyboards are more on the synthetic side maybe.
im thinking about the roland fantom-X series. i saw them in the store this year and was impressed.
X-A is cheap and basic, but still quite impressive compared to its equivalents in korg and yamaha.
http://www.roland.com/products/en/Fantom-Xa/index.html
next are X-6, X-7, X-8 which have the first ever full-color screen displays in a keyboard.
http://www.roland.com/products/en/Fantom-X8/index.html
it "looks" good on the surface. but wonder about the actual quality and final results though.