Audi R8 - Much better looking and up to date vehicle. Makes the M6 look like an 80's relic.
Porsche 911 Turbo - Screams ultimate sports car/luxury bordering on the exotic. Although classic in shape it's revised tweaks also make the M6 also appear old
Aston Martin V8 Vantage - This version dressed up looks more nouveau and stands out next to a bland M6.
For me the M6 is last on the list here given a choice. The interior also seems like a facsimile of a high end car, just not hitting the mark.
What's not "up to date" about the M6?? I assume you're referring to styling only (which is mostly subjective)?
Your comment about the interior also seems a little ridiculous as it's hard to see anything wrong in those pictures. I can say from the experience that the quality of the previous-gen M6's interior was sublime. I'd expect the same here.
No way is the 911 Turbo, R8 or ZR-1 a competitor for this vehicle. Those are clearly sportscars, while this would be cross-shopped with the RS5 and CLS/CL63.
Regarding weight: I'd prefer all cars to be lighter, but I don't mind using the GT excuse in this case. Just as I wouldn't mind using the Muscle Car excuse in the Camaro's case. For the GT, it's about providing all of the comfort, luxury, and even some capacity that a GT has over a more dedicated sportscar. That's the main reason I doubt anyone cross shops this with a 911T, R8, ZR-1, etc... In the Muscle Car case, it's about saving $$$ compared to engineering something like ZR-1. Vman is right though, people need to adjust their perceptions of what 'heavy' is and judge all these cars equally.
As for the M cars, I'd take the M6 Grand Coupe over this or the M5.