Today I could finally compare the two, and I must say: the stereo is really different between the two keyboards. the X50 sounds deeper and in headphones. To generalize, I'd say the X50 is fuller and maybe deeper, but the Kross is more transparent and balanced over the frequency range.
Pianos: I agree with what TWStone said in the post above this.
EP's: X50's may be a bit more solid, but not all that different.

wait!

edit: there's an FM E.Piano (A78) on the X50 that is gorgeous. Like a TX816 which was 8 DX-7's in a rack mount! And an analog EP that just says, "Play Me!" And the first EP, (A17) is also kind of normal in a good way, touching all the bases. For me, the Yamaha EP's are the best. Kross has a few cool ones, but I've yet to settle on a favorite Kross EP. Some have a bit of bite, some have plenty of sparkle, but seem thin overall. There are some funky ones that take advantage of the wah or vocal simulation, mostly wurlitzer ones (on the Kross.) But they all sound slightly 'far away' to me when i play them. But they are useful in many live situations - the better the sound system, the better I sound.!. Just takes some getting used to.

PS. Now some good news for the Kross fans: take the 002 Factory EP Program- E.P.Distortion SW2, and light both switches, which will turn on modulation, and turn off the distortion, and you have a pretty good Rhodes, in my opinion. (especially if you play it from an 88 note Stage Piano, and blend it with one of their dark ones with a bit of bell on it.)
Strings: X50's smoother, if you like smooth strings. (I do.)
Brass: Haven't checked yet.
Organs: I still like the Kross for organ, but the X50 has the older Korg gems.
Basses: Both great. Both massive. Kross's are moderner, incl some Dubstep
Pads: X50's very lush; Kross's also lush but more transparent, with a bit of a tendency to peak in the mids.
Synth Leads: Both good. Depends on taste, but I think there're more in X50.
Drums: haven't checked yet, but I'm pretty sure TWStone hit it right again.
I don't want it to sound like I prefer the X50, but the 4 3-purpose knobs are nice to have. The Kross is super-cool, the drum grooves newer and tastier, and Taylor Swift's not the only person who digs RED! - If I had to send one back, I'd probably keep the Kross, but I'd miss the X-50.