I'm sorry but if your looking for performance, the E55K is a monster compared to the 645 without a shadow of a doubt (when you drive it, you will feel so inlove with it you will forget about the crappy 645! ).
I personally like 6 series and i think they look good, but seriously in terms of comfort speed and handling, the E55 will win hands down. The 645 is just a standard car with a standard V8, the AMG is a tuned car with a tuned V8, there's a big difference! (I only say 'crappy' 645 cuz its seriously no comparison to the E55).
I think some test drives in order to make you more decisive!!
I personally like 6 series and i think they look good, but seriously in terms of comfort speed and handling, the E55 will win hands down. The 645 is just a standard car with a standard V8, the AMG is a tuned car with a tuned V8, there's a big difference! (I only say 'crappy' 645 cuz its seriously no comparison to the E55).
I think some test drives in order to make you more decisive!!