 I owned a 2003 320 SLK for the past few years.  A great car, but the 350 SLK is light years ahead of it.  Less side torsion for one.  Handling is without question better..big upgrade.   Instrumentation is spectactular by comparison.  Similar in layout, but the it's like going from old school to the real deal.  I wouldn't trash the 2003..it was beautiful...but this turns heads like you cannot believe...almost embarassing.