I mean, it's best to try them side by side in a store if possible, but if not:
sx900 has 4 speakers totalling 50w, the sx700 has 2 totalling 30w ; the sx900 speakers are *much* clearer and 'beefier'.
The sx700 has 400mb expansion area, of which approx 250mb is pre used.
sx900 has approx 1GB expansion area, of which the same 250mb is pre used.
sx900 can do audio styles, sx700 cannot. The sx900 has vocal harmony and vocoder. The sx900 has all the same choirs and voice samples as the T5, G1, G2, the sx700 only has a very limited selection of those (mostly the Oohs and Aahs, no jazz scat or wave cycled phrases). THe sx900 has sampled solo strings, the sx700 only has sampled ensemble (compare the solo violins & cellos to really hear the difference)
And finally, the sx900 is has revo compatible live drums, so it can use all the Genos styles (and a lot of the G2 styles as well, since the ambient drums were mapped to the same keys as revo drums). The sx700 uses the T4/T5 drum set, which means most current and future flagship styles do not work (CVP809/909, G1, G2 styles will work on sx900, not sx700, at least not without some major surgery)
There's more differences; if you're in North America there's no reason not to get the sx900 (in NA the sx700 vs sx900 price is 70/100)
But in Europe? about 60/100! (used to be 50/100, shockingly!)
Mark