Answer
I have heard the same. the slk is sleek and weight distribution feels the same on the highway, however in the city (such as los angeles) the drive quality leans quickly to the slk.
the spyder keeps its feel of new car handling for well over 12,000 miles. the mercedes for over 20,000.
i chose the spyder because of the dollar for dollar value comparison where repairs for the slk had costs much higher
and the frequency of repars were less often on the spyder.
one additional note, i am not patting my self on the back, but
(fortunately) i can drive
any car i choose; i have tried many many cars and i drive an eclipse spyder (2005 kalapana black on black manual), and i love that car.