If you are grand piano player and you need the most realistic sound then Kurzweil is your choice. If you really want a good synth engine (with sampling not as powerfull as Kurzweils but good enough for almost anything) with many preset sounds good and easy editing, cool present sounds, piano weighted keyboard (Pro X),nice midi realtime controllers etc then buy the Korg.
These two monsters are really different. From S&S engine to OS and from sound editing to sounding personality. It is not easy to give you an advice. Try 'em both and listen...
Until recently i had a Kurzweil 2500 in my home studio mainly to produce realistic piano sounds. Now i have Triton Pro (not X) with the extra piano PCM sample board but i found that piano sound is not as good as K2500. With a little editing on Korg's piano presets i found an interesting sound.

So the choice is yours. But believe me...
If you want one (maybe the only one) workstation synth with full extras buy Korg.