The difference is that BMW is faster in curves, but slower in straight, high-speed passages. The CL65 could drive ~350-360 top speeds, but will be quite a bit slower than the M3 in curves since it weighs quite a bit more and thus has less traction. It has the strongest brakes ever used in a "normal" car, obviously a necessity. The torque is limited to 1000Nm since fully open it would regularely rip off axes or wheels when heavily pushed.
Technically, BMW, Mercedes and now also Audi have practically no true differences. All get their parts from same suppliers/developers and these are sometimes even actually 100% same parts. They try to differentiate through company image and a different setup. Mercedes is all about luxury and comfort, even in their sportier cars. Not even in a new AMG SLK you'll ever complain about the suspension, although it's very sporty. The steering is a bit cheesy and effortless in all Mercedes, and that is the way they want it. BMW is all about being a bit like a race car - the driver is always sitting exactly in the middle between the axes, the steering is a bit harder but more sensible, suspension is usually sturdier as are the seats. You will drive faster in curves automattically, since you can controll the car better, but driving a car like this reqires a bit more effort.
Electronically, they offer the same toys. Both have shift-by-wire, both will get ESP2 now, both have air-suspension and so on.
As for being faster, look who won the most championships in the DTM, if that tells you something
As for being biased, I'm driving a BMW although they're competition. I like their setup better