Just a quick note about the differences between the MM8 and the MO8, and they are more than subtle.
Polyphony: 32 MM8 vs. 64 MO8 - Huge difference when playing piano and I'd consider 64 the bare minimum (and 128 is preferable) these days.
Sound:
MM8 uses "original" Motif sounds, while the MO8 takes its sound set from the Motif ES.
Action: The MM8 has "Graded Hammer Standard" while the MO8 has "Balanced or Graded Hammer Effect Action". Both actions are good but the GHE has greater sensitivity and authenticity than GHS action. If you're NOT a hard core pianist, you might not care about this difference.
Waveforms: MM8 has 70 MB , while the MO8 has 175MB.
Effects: The MM8 has an adequate but far more limited choice of reverbs, delays, chorus's but also how they can be applied is different than the MO8.
Sequencing: MO8 has it, MM8 doesn't. Even if you don't think you'll need it, it's more valuable on the resale market.
Given the relatively small difference in price ($400 USD street price, not MSRP), I'd likely take the MO8 over the MM8 on the ES soundset and additional polyphony alone. I'd consider the sequencing and other workstation features as a freebie!
Your mileage may vary.