I see a Prophet X to be like Ableton live & improvisation. And I see a Montage/Motif XF/Kronos/Kurzweil to be like a traditional linear DAW & composition. For some, one is a clear path.
Phil is amazing. Yamaha Synth Division is indebted to this long-term teacher. I was just reading some of his stuff today. I'm a Motif XF user. I had a good look at the Montage when it (& the OB6) came out, but decided to stay put. I was incredibly excited by it as an evolution to what I had. The learning curve with the Motif XF has surely been as steep as advertised. The existing various means of support are critical. My fundamental reason for the XF was for sample playback with zero lag. And I've grown attached to its sequencer.
Strangely, I learned synthesis from Phil on the XF, trying to make it be a Prophet '08. I got to real interesting places, only scratching the surface. And then, the Rev2 was announced the day after I shook hands with Uli, having just tried out my 1st 12 voice monotimbral, and identifying the importance of polyphony for my top synthesis goal. The Rev2 is addressing my real-time need.
Good luck on your call. ... Identifying needs, workflow, & thinking complementary seem to be good guiding posts. As is gut. ... I think Dave addresses the Prophet X vs workstation comparison pretty saliently in Nodes & Wires, Ep. 4.
One of my top favorite things about DSI is the thoughtfulness & courteousness of this forum. ... The Tempest forum could be a different hang, though. ... But there, beat is king.