The CLK320 is the other car I was looking into when I bought my 02' 330Ci a year ago. For me after driving about 20 or so different cars of both Merc's and BMW's the choice was clear. I like to DRIVE the car.
Both cars, being 6'ers were similar in performance. I'm not looking at specs but the Benz probably had the edge on torque because of the V6 configuration. It also "felt" like it was slightly peppier from a low speed 'step on it' get up and go thing but I'm fairly sure that was also in part to the suspension which deadens the road a bit more.
The ride of the Benz was definately more refined if you are looking for the Cadillac type of experience, on the other hand the BMW allows you to feel the road very well, all of it.
The interior of the Benz, as far as I was concerned, was a litter austere for my taste. I mean what's with control buttons that are the size of a door bell??? Slight exageration but I mean really, what's with that? Six black buttons lined up next to each other on a plank of wood! BUT the CLK320 does have dual zone automatic climate control instead of single zone.
The fit and finish of the car is as good as the BMW at least in the 2001-2003 models I was looking at.
I think the longevity of the mechanicals for both is pretty much a toss, but the electricals for 2002 and 2003 BMW's they were very highly rated at least from Consumer Reports perspective. I know a lot of people bash these types of guides/reports but at least it gave someplace to start from.
In the beginning when I set out to buy a car this round I was actually going out to buy an 02' CLK320, but after looking and driving many cars, In the end the styling, lines and feel of the BMW clearly swayed me to buy that car and I have absolutely no regrets. I still think the CLK series looks sharp too, but I like my Bimmer.