I'd love a Jupiter 4, korg Monopoly, or the DSi P08.
I had DSI P08 - I really liked the flexibility, hated the encoders (I guess you don't have to worry about that if you get the POT edition :P), the sound was cool for poly duties and FX, not so much for bass.. But it was lacking something in the sound for me (and I really really hated those encoders), so I sold it.
Bought monopoly and Jupiter 4 instead, later on installed Io midi kit as well. Never regretted it, the sound is soo much better (for me). I'm gonna say you need both, but if I only had to chose one, it would probably be monopoly.
- great 70's quality sound!
- super fast lfo and envelopes
- quality hardware, built to last!!
- excels at arpeggio sounds and fx (great for Kraftwerk/doppler effect/anthony rother style electro), chords also quite ok
- great sound, comparing it to jup4 I'd say more "liquid", "round" and "bubbly", also a little more "plasticky", but very useable in a lot of situations
- super fast lfo, nice envelopes for leads
- really fun, great for live performance (chord mode, crossmod, arp cycling through all osc etc.)
- very flexible, great bass, nice leads, crazy fx, jazzy chords..
For me monopoly is the best all around synth, super fun, great sound, can't go wrong with it. Jupiter is also great, very nice sound, but a little more limited. If you can, try them in person!
That said, I wouldn't mind owning P08 again (without encoders), but there are some synths that are higher on my priority list..
It also depends on what you're looking for - if you mainly need a poly synth, get the prophet, becase poly mode is more of an add-on to monopoly and jup4.