The sound routed through the main output will be the same for the same voice; but there are a lot of additional voices on the sx900, and the piano is more dynamic. Obviously the speakers are better, too, but the sx700 only has the choir "ooh ahh" voices, the sx900 has all the voice samples from the T4/T6/Genos. The sx700 only has the drum kits up to the T5, the sx900 has revo compatible ones that let you play back all the Genos/CVP809 styles without filtering out all the additional hi hat variants. The sx700 has only 400mb on board samples, of which almost half is already used by preinstalled sounds. The sx900 has the same, but with 1gb of available sample ram, that means you don't *have* to erase the presets. There are also a lot more T5 guitar sounds on the sx900 (the sx700 is based on the T4), include the ultra wide "twin" guitars where they routed 6 pickup poles across the entire stereo width, and it sounds amazing, as well as true flamenco guitar (the sx700/s970 flamenco guitar is the same as the concert Classical, the sx900 is quite different).
Bottom line though, if the piano, vocal harmony, speakers, choir, drums, installable sample library and future style compatibility aren't important to you, then you should probably get the sx700 (which is still a really good board, roughly equivalent to the s970/975)
Mark
edit: forgot 1 more difference; the sx700 doesn't have built in sampled solo strings (violin, cello) the sx900 does, *but* that can be mostly alleviated with downloadable samples.