Some owners and magazine reviewers have mentioned that whilst the CSL brakes (on a CSL or on the Z4M) are very good at stopping, they do run out of breath after a few good hard laps...
So far, replacing the SLK's pads or brakes has been a non-occurence for owners. Most have gained tens of thousands of miles on theirs (with track use as well as standard driving) and they have not had to change anything... My pads are still the original ones, and I've got 33,000km's on the car (and occasionally I do drive in a spirited manner).
I can't get any proper numbers on the CSL or SLK's 100-0 stopping distance (did a quick check in google, but nothing proper yet), but I do remember one review saying that the 55's brakes were very good, able to stop the car almost as well as the new composite brakes on the new Boxster, which is the best in it's class. Maybe if the SLK was as light as the Boxster, it would stop quicker? There is a "Nurburgring" setup available for the SLK with even larger brakes (composite ones) which might be even better... but not many people have that option as it only came out some months ago. I guess the CSL might also be one of the best in its relative class.
But the CSL and SLK are also very different cars

The CSL is definitely the "track" car, the SLK is just a cruiser