The SX900 speakers are excellent. The SX700 has a different system but I believe still pretty good.

Whilst I'm not a huge style user the ones I've tried so far have all been fine.

Ultimately you really need to try both keyboards and see which best matches your needs.

I now have the SX900 (replacing Tyros 3) rather than the SX700 because I needed the sub outputs and external video, the sound quality on both is excellent.