My thoughts about SX900 vs SX700... and I'm not writing this to convince myself into what a great decision I made. It's more a personal opinion for those who are considering either of these two keyboards.
Is not a surprise that SX900 is better. The question for a potential buyer is, in which departments is better and how much difference that makes in everyday usage. SX900 has almost double price of SX700 and so one would expect SX900 to be substantially better than SX700 -which, in my opinion, is not the case.
Both have the same set of voices, where SX900 only has a few more. Are these additional voices that exceptional? And how important are these particular voices for potential buyer? -I mean, cello isn't something we use that often. And if someone really needs particular voice, it can usually be found on internet.
Revo drums... I can't comment on that, because I don't have them. But honestly, drums on SX700 work just fine for me. And again, if there's a case that particular percussion is missing or is not "the right one" for particular song (i.e. tambourine is embarrasing on all SX'es), then we can download the sample.
At the end, no matter how many voices we have, there will always be a missing one (i.e. violin can be played in many different ways) and we will need to buy it or create a custom one -that's why that option exists.
What I do miss on SX700 and SX900 has it, is bigger user memory for voices. I'm not a voice "collector" and I'm quite selective, but 400MB really feels small sometimes. For those who are buying first arranger keyboard: how limiting that is in everyday use? Let me put it this way... There are already 1000 voices built-in. So, how many additional do we really need? We don't need all the voices (that we collected) all the time. User memory is meant for additional voices that we actually use. And if we look that way, then I think 400MB is sufficient.
For sake of completeness: how much memory is needed for single voice? Depends on voice type... it can be 5MB or 100MB (average being about 20-30MB). Means, we can add about 10-20 voices on SX700.
SX900 has few other hardware features which don't exist on SX700. Vocal harmony, for example, which can be important for live singers. But that can be bought as separate device anytime later.
Another feature I like on SX900, is additional Line output, which can be useful for recording. The problem is not that SX700 is missing that additional output... the problem is, volume level of Line output on SX700 can not be separately adjusted (independently of main volume).
And that's pretty much all. Is it worth the price difference? Is a question everyone should answer for himself. Sure, if money is there, why not? I mean, nobody will say no to additional features. On the other hand, SX700 already has capabilities & features that majority of owners never use. One is for sure: is a great hobby

Bogdan
PS: agree with
Lacko. But truth is, because of various reasons, big majority never decides for additional speakers. And so, the quality of built-in system can be quite important for those. But yes, if sound quality is the only reason when deciding between SX900 and SX700, then pairing SX700 with external speakers is smarter solution.