Although quoting oneself could be considered egomaniacal I addressed this comparison in a post a couple of weeks ago. Honestly, I would recommend a 987 base instead of the 986s at the price range you're considering. I would rank'em like this:
987 Base
350 SLK
986S
280 SLK
Whole thread:
Care clinic for you San Diego types!
My previous comment:
"I got the SLK awhile back on a a 3-year lease. Back when I was car-shopping, I was going to get a 650 convertible or an SL500, but after doing some test-driving, I liked, but was not overwhelmed with either. So I basically split the payment and got two cars instead (for various reasons, I do a boat-load of driving). Driving two roadsters may seem borderline-retarded at first but the comparison between the two cars ends there, in my mind. The Merc is also well-optioned, where the Box is not, but they are still two distinctly different vehicles.
I drive the Merc when I don't feel like manually shifting, having the six-cylinder drone right behind my noggin, or there is the distinct chance of rain. Otherwise I'll drive the Box. In an answer to a question no one asked, my comparison between the two would be:
Merc:
Has some standard features that should be standard on the Box
Air-scarf is awesome (blows hot or cold air on your neck, may cause an erection)
7 speed automatic is pretty good, must drive aggressive in sport mode or shifts very rough
Better cockpit protection in wind
Crap standard stereo, call it a tie with the Box
Vinyl windscreen works ok, but severely hampers night vision/looking out for cops
Feels a little more torquey, not a severe difference unless in fifth gear where it's much better
Automatic hardtop is an engineering marvel in motion, worth the price of admission
Steering wheel adjustment is much better (it's mechanical)
Standard seat adjustment is much better
Cupholders are retarded
Trunk space, particularly with hardtop down is a joke
Better standard OBC
Best looking rear-end on a car....ever
Better looking standard rims
Box:
Better seats overall, although Merc's is good
Coolest cupholders ever
More trunk space
Constant drone of engine behind head with top-up is eventually grating. Some would disagree.
Retractable spoiler - high level of coolness - just wish I could see it
Better headlights
Crap radio reception
Wind screen rattle - still
Better handling, although not the wide gap some might think
Better sounding exhaust/engine note, although it could use a little bass, which the Merc and Boxster S has."