As usual you have to justify the expense of an upgrade based on the things you think are most useful.

I justified the upgrade from a 2000 to a 3000 solely on the convenience of having significant on-board internal storage. I use a lot of midi files and eliminating the "floppy shuffle" makes a huge difference to the smoothness of the performance and was worth the money on its own. The USB capabilities were the icing on the cake in this regard.

As a slight aside, I consider the removal of the Smartmedia card (more exactly the amount of on-board storage you get) to be a negative feature of the 500/700/900. Given that Smartmedia is practically obsolete I was not surprised, but given the very low cost of flash memory I was extremely disappointed to find that the 500/700/900 did not have significantly more internal memory (versus the 3k) as some form of compensation!

The megavoices in the 3k are useful but you have to program your existing styles / sequences carefully to gain full advantage, although Yamaha's megaenhancer software does do quite a good job of conversion.

Having played a T2 the SA voices are very pleasant, but I don't do a lot of pure instrumental work so the extra realism, remarkable though it is, is not a big factor for me.

I'd agree that the s900 is worth the extra over the s700 and is worth saving up for. If my 3k blows up I'll be looking at a 900.