Pkoslow, thanks for the links to the pdfs.
Here is my 2 cents on the M6 and the Concerto.
The Concerto provides 10 more watts per channel, and 2 additional non-amplified zones.
When used with a Nuvo T2 AM/FM/XM Tuner the keypads can provide station call letters, name of the artist, etc. and provide function feedback when you press a button.
Not only will it integrate with the T2 Tuners but now Nuvo has just introduced a music server and soon will have an iPod dock that can display information to the keypad!
Nuvo also combines all the Cat5 cables into one wall plate, which means less cable clutter.
Another consideration I think is worth mentioning is that the tuner on the M6 is integrated with the Amp. I am not crazy about that. Kinda like having a DVD player built into an expensive plasma, if the the player goes the whole unit has to be shipped back for repair.
One feature of the M6 that I did take notice of is the contact closure, perhaps used with macros to control drapes, motors, etc. If you already have a HA system then it probably won't be used.
I guess the biggest factor in this comparison is price, Smarthome sells the Concerto for $7000 without the tuner! Of course you can pick it up much less than that, but I imagine the same holds true for the M6 when it does hit the market.
If the M6 price is half of the Concerto, then it will command the market for this kind of technology. However if money does not matter the Concerto wins my vote.