If you are thinking of a W211 than seriously think about the E320CDI. I have the pleasure/honour of driving bothe the X5 and E320CDI and can say both have admirable qualities. The E320 CDI however is truly an amazing piece of engineering. 369 lb of torque at 1800rpm, extra urban mpg of 39 and urban 28. 780 miles on one tank and absolutely no smoke on startup, no clatter or smell. According to MBUSA, 0-60 mph at 6.6s! If you're in a non carb state, buy one or we wait until ULSD comes here.
The interior appointments are the best I have ever seen with close competition with Audi.
Hope this helps. I can't speak for the C class but did find it mildly "plasticky" and lacking in real refinement IMHO.
Jay

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Bring the 3.0D and they will come!