Here is an interesting answer and some perspectives from Yamaha's Bad Mister that was posted on the Yamahasynth.com Montage forum:

"...at the end of the day is the difference between the CP4 Stage target customer, and the Synthesizer (Motif /Montage) target customer. And why SCM is found in a piano based product, and AWM2/FM-X are found in the Montage. SCM, while it has a few user adjustable parameters, Modeling can be quite hostile to those without a scientific background; the Technology in the Montage is very hands-on. Even more hands-on from an editing/customizing point of view than previously possible.

While the tech in the CP4 Stage is next level stuff, and plays extremely well, and records perfectly... On recordings it is indistinguishable from an acoustic - if you weren't told you wouldn't know. The fundamental Rhodes, Wurlis are completely nailed and it shares the appropriate VCM effects for these iconic electro-magnetic systems... Recreating the fundamental tones and the effects of their era!

Which has a better piano sound? - is always a personal taste thing. And frankly, my co-worker, Blake and I were laughing about this because -how cool is it to have to choose a favorite smile

Playing the pianos in the CP4 Stage, and then in the Montage (which by the way can include them all) at NAMM in one Live Set I had the "S700 for Montage", the "Full Concert Grand", the new "CFX Grand", and the promotional Bosendorfer "Imperial Grand" - each inspired a different mood, each was worthy of being a favorite. Each ultimately tweakable.

Typically, when playing a sample-based piano emulation, you are working in the wheelhouse of the technology. Samples are really good doing percussion instruments versus instruments that are blown or bowed. The difficulties of the piano, being a very complex percussion instrument are well known... In order to make the piano "speak" well, you have to sample at multiple velocities. It is easy to understand why... The behavior of a hard strike is different from the behavior of a soft strike at the string level.

Just as hitting a crash cymbal (another percussion instrument) is very chaotic when struck hard, and not so when struck softly. Just adjusting the volume of a hard struck cymbal does not "fool" you - you can tell it was struck hard even played back at a low volume. It sound like a hard strike played back at a low volume, not like a softly struck cymbal! Same goes for a piano string, the chaos or lack there of is obvious, to the ear/brain. With piano it's more subtle but you notice it... When its not right!

SCM avoids this entirely as the technology is not simple velocity swaps. This is something that you "feel" over time when playing the CP4 Stage, it is sweet. However, when velocity swapping technology is used the compromise is always in the number of velocity swaps and where they occur - *where* to concentrate your effort. For ballads you might want more behavioral difference at lower velocities, for rock you may not require subtleties like this, with AWM2 as available in the Montage allows scuba divers a huge advantage to "detail" their playable sounds like never before.

We don't have all the answer yet, but comparisons between CP4 Stage and Montage should NOT be done exclusively by which has "the best" piano... But with the other features and product focus clearly in mind. In other words, what else do you need it to do?"

So the bottom line, if I read BM correctly, is that SCM (modeling) technology is not at the point that can provide the varied, deep and usable parameter tweaking that is part of the Montage synthesizer design and philosophy.
That said, still wondering when/if modeling and other Yamaha synth engine tech will trickle down again to this or other Yammy synths