I've listened carefully to the above video a few times since it first came out, and it seems that the Montage sounds a little better. I know that the Montage is a touch louder, but even ignoring that fact, its sounds are a tad warmer and sweeter. So, perhaps people who want the absolute best sound and can afford it are better off buying the Montage.
Funny how Yamaha always says the MODX has the same engine, or the same sounds, and never mention that it's not the exact same sound quality. Many people hear the same thing as I do, so I'm not imagining things. I've read somewhere that the MODX does not have a certain component that the Montage has. I believe they call it a "PAC".