My Personal Opinion
Sounds:
If how the sound is articulated is important to you then Yamaha takes the lead, (Just remember there are only a few of them) however if like me you like something that emulates the Real instrument (Which takes more then just articulations) then the Korg pulls quite a bit in front. (Plus it applies to many more voices)
Styles:
If you want everything including the Kitchen sink, then Yamaha is easily the best, however if you wish to choose your own kitchen (Or you come from an organ playing background) then the Korg wins hands down.
Vocalises:
T C Helicon is still the best. (Inbuilt or external)
Overall:
The sound system in the Yamaha rounds off and compresses all the sounds produced by it, thus giving a studio produced CD like sound.
The Korg sound system lets the keyboard sound come alive, thus it feels more like you are listening to a live band. (My preference)
OS:
The Yamaha wins hands down for ease of use, the Korg OS however allows you to do a lot more to customise your performance for your own requirements.
As mentioned at the beginning, the above comments are my Personal opinion only, and as everyone is different, only you can make the final decision as to which you prefer.
Hope this helps
Bill
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English Riviera:
Live entertainment, Real Ale, Great Scenery, Great Beaches, why would anyone want to live anywhere else (I�m definitely staying put).