I think it would work well for you, I'm assuming based on only keeping your cars for 2 years that you are leasing (although you could be selling them I guess).
The 550 should be a good upgrade as the interior is more spacious and the N63 engine has plenty of power on tap and should satisfy your need for more power.
Only possible downside is that if your 6 has xDrive (wasn't specified in the post), the switch to electric steering will feel weird, since xDrive models of the 6 have hydraulic steering, but if you have a RWD model the steering should feel familiar.
If your looking at Mercedes I reccomend you look at the E53 (the replacement for the E43), I have the CLS variant of the E53 and its a very good car. It doesn't have as much power as the 550, but I overall prefer its driving dynamics to the 5.
As for the Audi, the current S6 isn't out yet, so I can't speak for it, but if its similar to current Audi's the transmission tuning will likely not be very smooth and laggy, and the touchscreens will be very difficult to operate without looking away from the road.
Good luck, hope you find what you're looking for!
