My advice would be to wait and see what Roland comes out with at next month's Winter NAMM exhibit.
As for controlling a MIDI module with any controller, keytar or otherwise, it's simply a matter of reading the manual and programming the knobs and sliders to send the right MIDI messages the module is expecting to see. It's not really that hard.
As for the extra octave and the lousy 256 voices? I agree it's not worth an extra $800.
You could just as easily take any 4 octave controller and add a neckstrap to it to make it wearable. The only thing you lose is the ribbon instead of a mod wheel and pitchbend.
And since you're a sax player maybe you'd be interested in buying a custom built Kimophone
http://kimolobo.com/
Might be pricey but it would definitely be a unique hook for a stage act.
Gary