RPiM5 is correct & Audi isn't coming out with a RS6, which would be a true competitor for the M5. I picked the RS7 b/c it's got room and it's coming to the states. All of the Audi S models aren't competitors for any M car.
The 2014 E63 looks like a beast, but Mercs just seem like they are trying to hard to be something their not (i.e., trying to be a performance brand when they are really a luxury brand).
See, this is why I couldn't really come up with a real competitor and why I purchased the M5.