hi Leonh,
Before I continue, I have SX700.
Just to clarify, because there can be misunderstandings about keyboard memory. Yes, SX900 has 4GB and SX700 has 1GB of user memory (which is 4-times less). But there's also term expansion memory, where SX900 has 1GB (1000MB) and SX700 has 400MB of it (2.5-times less).
Which of that is more important? Depends, but my opinion is, it's expansion memory.
User memory is the place where you save all your keyboard settings, custom styles and modified styles settings, modified voices settings, etc. For illustration, single style is usually about 80kB (other single settings need less space). Means, in SX700 user memory, we can save thousands of styles and still have enough space for various settings.
However, we can't save external voices into user memory. Voices can only be installed into expansion memory. The "problem" is, single voice can take 50-100MB or more (depends on quality/complexity) , and having only 400MB on SX700 (1GB on SX900) doesn't feel that much anymore. And before asking, yes, there exist better voices than many of those built into SX keyboard.
Having only 400MB, is my biggest (and actually the only) complain about SX700. Anyway, price difference was too big and not really justified for me -considering that I don't miss any other features which SX900 has.
I just pointed out difference that's important to me, but that doesn't mean SX700 can't be "seriously" used! After a while, we usually carefully choose few best voices we use most often. For the rest, there are plenty of them built in keyboard.
Amplifier/loudspeaker quality... SX700 is more than good/loud enough for home use and I can't imagine SX900 is much better. In both cases, if better is needed, some external audio system must be considered in my opinion.
Yes, I would buy SX700 again.
Bogdan