Agreed, I wanted a 55 from the beginning due to my prior experience with how the excellent I found the M113 motor to be in my old C55. I did however try a 2008 CLS63 for sale near me before I bought my CLS55, and I hate to say it but the 63 just wasn't as... I guess you could say raw and engaging to drive as I find the 55 to be. It was fast but lacked the violent feeling of acceleration the 55 delivers. The best way I can describe it is it was like the previous generation BMW M3 and M5, sure the M5 had a V10 and 500 something HP, but its torque was seriously lacking at only 383 ft/lbs. The same can be said of the W219 CLS 63, a properly fast car, however lacking in the pin you to your seat sensation department. If I were you I would find a clean, low mile, 1 owner CLS55 AMG. I can promise you it will not disappoint. If you have the cash hell go for a W218 CLS 63 like the other guys said, that is an awesome machine as well. Whatever you decide on purchasing be sure to get an MB dealer to pull the VMI for you and also have the car thoroughly inspected, post any questions here, and do not be afraid to walk away from a car.
Happy car hunting let us know what you decide!