Technically true; however the additional Revo based live drums on the sx900 are superior to the Tyros 4/5 drums that already exist in the sx700 (and yes, those samples are identical). Additionally, if you want future styles from CVP/Genos, they all use the new Revo drums (and don't conveniently convert to the old drums that the sx700 already has... yes you can use Jorgen's software to remap Revo drums, but it's not quite the same)
So, yes, the sound quality *of the same sounds* on both models is the same; but the sx900 *exclusive* sounds are noticeably higher quality.
With the exception of the grand piano; which is better on the sx900.
So if you don't need the better piano, strings, drums, the addition of solo strings (violins & cellos which are sampled on the sx900 but not the sx700) and sampled pipe organs (sx700 doesn't have sampled pipe organs), and more brass and guitar sounds (including the amazing twin guitars which alternated panning of pickups 1-6 panning left to right alternating), then then get the sx700.
The sx900 is essentially a Tyros 5 minus the S.Art2 sounds, with the addition of Revo compatible drums.
The sx700 is esentially a Tyros 4 minus the S.Art2 sounds.
Mark