Are you buying new or used? Buying just for the summer or do you plan on keeping it for a while? Will it be a primary car or only a good weather car?
If you're considering a C4, I would also look at the C2. Lower price, less expensive maintenance, and it's lighter. Used Tips are cheap too.
M3 is good....good balance, power, pricepoint etc. What about a 330 convertible?
Depending on how you'll use the convertible, some considerations are:
how slow the top goes up and down
visibility of the top
interior noise
Personal impressions:
CLK has GREAT resale value, visibilty is poor, and the power top is SLOW
BMW is good choice
Porsche C2 is a good choice
Audi has poor resale value, not as sporty as BMW or Porsche
SL is a GREAT car (style, power hardtop, etc) but poor resale value