The 600 and 55k have completely different characters, they are chalk and cheese.
Make sure you know what you want from the car before you set your heart on either.
The 55k is loud and proud, slightly stiffer suspension and more supportive AMG seats.
600 bi turbo on the other hand...silent missile. Most comfortable car on the road and would leave pretty much anything else behind on a straight open stretch of road.
The 600 will have higher running costs than the 55k. Fuel consumption will be similar though.
I will always argue that the 600 bi turbo is the quicker of the 2 cars in a straight line even though on paper they are identical.