I think very highly of the ARP Odyssey sound, but there are so many other modules available. From my monophonic synthesizer searches, I'd say the Hypersynth Xenophone (http://www.hypersynth.com/xenophone.html) is the most impressive. But again, there are so many others. Why have you decided on only the two?
ARP mostly because of my love of early John Carpenter scores which used two ARP Avatars (which were essentially Odyssey modules) and sequenced them while the Prophet 5, 10 and ARP Quadra did the leads/pads etc.
The Two Voice Pro because that sound is massive and the fact it's two synth engines in one with a pretty advanced sequencer is incredible. It would be nice to get something from all the masters (Smith, Moog, Linn and Oberheim) but it's also a huge investment and it's $5k for something that I might have to ship back and wait a while for it to be repaired OR even worse, if something tragic were to happen and suddenly I don't know the status of the company or my repair. YIKES!
My set up that I'm going for is
Prophet 6 (which I may also add a module version down the road)
Possibly a Prophet Rev2 (or an Alesis Andromeda in great shape or if ARP reissue the Quadra)
Moog Sub 37
Either the ARPs or Oberheim
An external sequencer if I go with ARP....possibly a Doepfer Dark Time.
Tempest Drum Computer.
I also have a mint condition Kronos which I never use, a Kurzweil 2600xs and a Roland FA 08 which I use for compositions and more classical/jazz/funk sketches. But I try to keep those separate from my synth sounding oriented stuff.