Piano presets on arranger keyboards are a joke. They do an ok job when camouflaged within backing tracks, but play it bare and they fall apart. Play a real acoustic piano and then play the sampled version - the difference is readily audible.
I have compared the piano preset on these three top arrangers:
Yamaha Tyros 2
None have a satisfactory piano sound. Of these three arrangers, the best piano preset is the Grand Piano X (without reverb) - the first preset on the Roland G70. The next preset, the Stereo Grand is washed in reverb - very unnatural. So, if all you care is the best natural grand piano sound on an arranger, go pay $3,500 and get yourself a Roland G70.
The Yamaha Tyros 2 takes a distant 2nd place followed very closely by the Korg Pa1Xpro in 3rd place. I have not heard the SD1 or the ProMEGA and cannot comment on them.
For better quality piano sounds you would have to move up to the Roland KR series or the Yamaha Clavinova series. For a really good piano sound, try the new Yamaha CLP-280 or the CVP-309GP ($14,000). They both have the natural wood keyboard mechanism with interlocking levers and weights. Good action with a great sound. Still not the real thing. To get that you would have to move up to a Yamaha Disklavier Mark IV ($35,000 plus).
The Grand Piano sound on the Yamaha CVP-309GP was my reference for arranger pianos till I heard the Korg OASYS. You must reboot the OASYS to load in the half gig piano sample. This is the finest piano sample I have heard on a keyboard that comes closest to a Steinway. The 88 key Korg OASYS ($8,000) is my new reference. I hope I could buy that some day!
Having said that, nothing compares to actually playing a real acoustic grand. Even between acoustic grands there are characteristic tonal differences amongst a Steinway Model D, a Yamaha C7 or a Mason Hamlin. You can tell a Yamaha C7 by its brash and brilliance. The Mason Hamlin will have that extra deep and rich set of overtones. These are trademark sounds that one immediately picks up. Both sound different than the unmistakable sound of a Steinway.
After all, it boils down to simple user preference. It is a personal thing.