According to mbusa.com, the E55 AMG does 0-60 in 4.5 seconds. I can believe the 4.2 second time assuming they did a brake-torque launch, which launches the car well but is not good for the tranny and the brakes.
The BMW M5 has the 507hp, 5.0 liter V10, naturally aspirated. I think the E55 AMG is stronger down low, but the M5 is a high-revving naturally aspirated engine that developes 38hp more than the E55 and is stronger up top. Also, the new M5 weighs almost 350lbs less (4,087lbs vs 3,750lbs) and has the SMG III transmission working for it (faster shifts and shorter gears to keep the engine in its power band).
I am a HUGE fan of the E55 AMG, but I think the new M5 is faster and handles better (I can't speak for braking because the E55's 8-piston front calipers look like MONSTERS). Either way, my decision between the two is EASY: BMW M5 all the way baby!
Mercedes has a great car with the E55 AMG, along with all the other AMG cars, but all Mercedes vehicles suffer from the same problem: as good as they are, they always have such a serious (boring) personality! The car isolates you from the road, the interior is all business, and the car does not reward you (with good feelings running through your body) for pushing it hard. Also, I think that the SMG transmission is the PERFECT transmission for the new M5. It is faster on the track and the lack of a real manual does not let you abuse the transmission, which would be a pretty easy thing to do with 507hp under the hood. And these cars are luxury sedans and sports cars, so I think the SMG transmission also offers that true manual "feel" and performance without the impracticality of a clutch (such as all those business people who have to deal with traffic every single day)....best of all worlds.
As you can see, I did not hold the E55 AMG's automatic transmission against it (I only mentioned how the SMG is superior to it) because Mercedes has "perfected" the automatic transmission in that 0-60 and 1/4-mile times between a Mercedes auto and manual is a difference of literally no more than 0.1 second.
As far as the RS6 debate, let's stick to one RS6: the last model before the brand new 2005 model, with the 450hp biturbo V8. Great car and very fast but suffers from two things: the automatic transmission and the Quattro AWD. Fortunately, this car weighs in at about 4,024lbs so it does not suffer from traditional Audi "chubbiness" but that Quattro AWD really slows the car down a lot (but as much as Quattro slows the car down is as much as it helps the handling). It's a true 50/50 power split and without Quattro AWD and auto transmission (unlike Mercedes, Audi auto trannies do take an acceleration hit over their manual trannies, but supposedly biturbo engines work very well with auto setup because the engine never actually "stops" working like in a manual but this is all subjective, I'm just drawing my own conclusion in that auto makes it slower as that is typical Audi fashion), the car would easily do 0-60 in a SOLID 4 seconds flat (but this is my own hypothesis, obviously not proven).