Very good summary David. This is exactly what I'm thinking about for the Synchron Pianos. Note that their Steinway and Yamaha are more like a modern concert grand piano sound, while their Bluthner and Bosendorfer have very different, intimate sound. Personally I feel the dynamic range of the Bosendorfer Upright a bit limited. Wondering how is the dynamic range of the Modern U? After a few months, is the Modern U still your favorite?
Currently, I find myself practicing more with the Modern U, but it really depends on what song I want to play when considering a favorite sound. Right now I'm working on an arrangement of Amazing Grace (Praise and Gospel Course with Duane Shinn) and the Modern U is perfect because of the style Duane has us playing it in (a lot of triplets and blues riffs).
Although, I have a intermediate rendition/score of that same song from composer Greg Howlett, and his slow and contemplative arrangement (IMO) lends itself more the the VSL Concert D. So it really depends on what you are trying to communicate with a particular song and what sound touches your emotions more when playing it.
The Modern U has surprisingly good power for an upright, but it can never compare to a 9' concert grand piano. If I had to choose just two VST's they would be the Modern U and the VSL Concert D.
I think a large spectrum can be covered with those two pianos.