I've been very busy catching up with work that I had to push aside for the last two weeks while getting the B6, taking a leisurely drive up the California coast, and then having to deal with a family emergency, but here's what I can say so far.
Bear in mind that I am not a racer, so reasonable minds may differ as to my evaluation.
The M6 is great fun. We live about 20 miles west of I-5, and the road we take is very curvy for those 20 miles because it follows along the side of a river. The car feels like it rides on rails, even in comfort mode. The car responds wonderfully to steering input. It doesn't take much input to get the car to do what you want it to do. In that regard, it kicks ass on the two AMG cars we owned before (an '00 E55 and an '06 CLS55). The negative: road noise. We regularly drive over Siskiyou Summit on I-5 at the California/Oregon border, and the pavement is somewhat exposed aggregate. It is LOUD! Even worse than the AMG cars, but not as bad as my '06 Z06. The M6 is somewhat closer to the Z06 in handling, but I had more confidence in the Z06. I had no qualms about taking 35 MPH curves at 70 MPH in the Z06, but I'm not quite there yet with the M6 (getting closer, though!). I don't have such confidence with the B6 for the reason I stated previously - handling is much more numb, and I just don't have confidence that it would hold on to the road if it weren't for DSC.
The B6 is definitely the car you want if you like luxury. It's the car I would want to drive clients in. It is quieter (especially with tire noise as noted above), and the seats are more comfortable. My butt at my right sit-bone gets very sore sitting in the M6, even after fiddling with the seat settings for some time. I had to buy a small cushion to keep comfortable. I had a similar problem with the Z06, but those cars were notorious for flimsy seats. I never had comfort problems in any other car, even rental cars. No problem at all in the B6. And shifting from one gear to another is smooooth!
Another big difference (at least for me) is acceleration. It may be because the M6 has a manual transmission (the primary reason I got the car), but I have to drive it like a Porsche - shift it from sixth to third and get it into the higher RPM's to get it to go. Even then, it begins to wind up and then surges big time. (Perhaps the driver needs to increase his level of skill?
) I much prefer the B6 in that regard. Acceleration seems to be much more linear and gives me that Holy S#!t!! feeling right from the get-go. Perhaps the best way to describe the two is that the B6 is more like an autobahn car, and the M6 is more like a track car.
Don't get me wrong. I absolutely LOVE both cars. But to answer the question posed in another thread about whether people cross-shop M6's and B6's, they may cross-shop the two, but I think a test drive in both cars will make it very easy to decide depending on where one's preferences lie.
I hope that helps at least a little.