According to the experienced pianist, The sound of Grand Piano of S700 ~= S900/S910 ~= T1234 They are all very very close. (Just for the Grand Piano voice.)
The more you pay, the more and better sound you will get. That's how Yamaha works.
If you just play piano, S700 is good enough. If you want to go into arrangement, then buy the highest model that you can afford.