Based on multiple testimonials and test reviews, I'd assume the M5c to be the best technical capable vehicle that builds more speed under most circumstances, but the E63s will most probably be more dramatic to drive.
My experience with the M5c was that it felt a bit underwhelming at first, until I looked at the speedo! Then I felt humble and realised this car is incredible awesome. Never have driven a E63s but I imagine it's not far off. And what matters then are not the differences in laptimes, but the differences in feel.
Currently, the only MB which could make me switch is probably the E53, since it's a better proposal than both the 540i and M550i. In the top segment, I would probably still bow for the mighty M5c.
