The Benz is horrible. I've never been able to find an angle that I like. They keep calling it a coupe. It doesn't look like a coupe and it looks terrible as a sedan.
The M6 is alright except the fact that it is missing 2 doors! The M5 is a much better value IMO. (even then, the styling is not for me...)
The STS-V looks sensational inside and out, there's no getting around it.
I think the exterior of the Benz is gorgeous, but the interior doesn't work for me, especially that weird "lizard tongue" shape that wraps down the center stack onto the console. Also, the wood used in the interior has got to be the phoniest, cheesiest looking real wood I've just about ever seen in a car.
As for the BMW, 90% of the exterior is fabulous, but then you get to that horrific Bangle-butt rear end, and it's aaaaaaaaaallllll over for me.