It's interesting that, although they are the same car, they recomend different part numbers for the SLK32 and the SRT6. They show the Heavy Duty for the SRT6:
Bilstein, Innovation and High Quality Since 1873

and all 3-types for the SLK32:
Bilstein, Innovation and High Quality Since 1873

It would also depend on what kind of ride you are looking for. I'm sure you can cross the HDs to a Sport if you wanted.