I've driven both! Never owned either. Performance wise.......the SL55 moved out very nice and you could really feel the power. Now, the SL600.....that started to scare me. The 600 just kept on pulling all the way to 100mph +! At about 80mph, it felt like it was pulling away from a traffic light! I was very impressed and......I own a Z06 Vette! Handling was very similar with either car. Both cars have an adjustable suspension and also, sport suspension buttons. You can firm up the ride for better cornering in both cars.
I'm presently looking for an SL600 as we speak! In addition, if you want to modify the car at all by getting the computer reflashed.......you get much more power out of the V12 than the V8!