Hi,
The biggest difference for me between the SX700 and SX900 is that SX900 has all the new Revo sampled Pop, Rock, Jazz brush kits, the new EDMKit, TrapKit, 80sPopKit, 80sR&BKit and SchlagerKit kit along with other perc instruments and also several brand new MegaVoices for eg, the twin-guitar strumming sound from Genos in styles (missing from SX700 and all previous S series models).
Bonus: In my opinion, the new drum kits make SX900's styles sound better than the Tyros.
Along with several other stuff like Chord looper, missing aluminum brushed finish, missing additional subwoofer (10W X2), etc. these are the main stuff I see missing on the SX700. It makes a huge difference in overall sound quality. The reason why I spent an extra to get SX900, worth it if you're really into it/or have a taste. I thought you might want to know this since the stuff I mentioned above (missing new drum kits and MegaVoices) isn't mentioned anywhere.
But either way, both SX700 and SX900 is a massive upgrade from E343. I upgraded to SX900 from my older E413.
Regards.