I have to admit I was partial to MB even for top down motoring...until I got my Boxster back in 2000. Still have MB but dumped all but the ML350 SUV, which I can't complain about. I did test drive the SLK350 after they revamped it last year just to compare it to the SLK you are considering. That 7 speed manual 3.5 liter is snappy and I would test drive it again if I was in the market to replace my Boxster, but no way I would take the SLK230 (smaller engine older body style) over a Boxster. My personal test has always been this: You know the yellow speed limit signs that they post before you get to a sharp curve in the road that kinda recommend a safe speed for negotiating the curve? Well just double that figure and take the curve. So if it says 35 miles per hour, take it at 70.
You'll like the Boxster going around that curve over any MB everytime!