Don,
I have an SD1 and an XD3, so I think I can comment here.
The basic difference is that the SD1 has double the ROM (48 Mb vs 24 Mb) and this means not only more sounds (especially organs and saxophones) but also better ones (the acoustic piano is much better on the SD1; on the XD3 it sounds kind of synthetic because the samples are shorter). On the other hand, on the XD3 Ketron has made a step forward improving the OS and the DSP (which is now more silent). Some sounds (like the electric pianos) have been resampled and are cleaner on the XD3. The XD3/XD9 lacks the hard disk (which is optional) and the sampler. Also, there are less buttons and the menus have been simplified, with some sounds (like flutes and saxophones) grouped in the same category. On the plus side, the XD3/9 has a feature called drum remix (you can play a midifile using the rhytm section of a style).
If you use the arranger just to back up your singing the difference in sounds maybe would not be relevant to you, because the styles are the same, but if you like to play a solo every now and then (and I know that you do) you would be happier with the SD1 (and this applies especially to the acoustic piano, the organs and the saxophones).
Ketron is about to release a new OS for the SD1 and this will hopefully include new features, already present in the XD9.