I agree with Ashman
All things being equal, buy the one that makes you grin the most and longest.
The lobbyists in my firm represent some of the air quality management groups in this part of California so I've been exposed to the diesel air pollution issue somewhat. I'm not sure about this with passenger cars, but I do know that commercial diesel vehicles are the source of a significant amount of the junk in our air around here. That aside, I previously owned a 240D and a 300D and they were both terrific cars. I sold the D and bought an E simply because of the acceleration factor. (Hottee18 is gonna kill me...)
If the air thing matters to you, you might check out that aspect of your purchase.
Someone is going to comment that my 300E, pound for pound of particulate matter per gallon burned, is just as dirty as my 300D was....
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban