I'm not sure you could make the extra cost of a diesel worth while at all. I'm going to use the Canadian figures I have on hand. Take 2 cars in the same size class, Hyundai Elantra (for the sake of I don't know of a more efficient conventional gasoline engine car, non hybrid, non turbo...etc) and VW Jetta TDI. In Canada they're rated for 58 and 61mpg(imp) respectively on the highway. That means the VW does only 5% better on fuel despite the extra cost of the diesel engine and the fact that diesel often cost more than gasoline.
The way I see it, a diesel car would have to do a lot better than a current Jetta to be worth while and for myself to consider buying one. Gasoline engines have improved in quality, durability and efficiency quite a bit in the last decade. I'll add to the fact that I live in a place where the temp can often drop below -30c (-22) and stay that way for a couple weeks, and even an occasional unholy -40c (-40f). A diesel car should start ok in that if plugged into a block heater (maybe). But what about those occasions where you stayed somewhere over night and don't have anywhere to plug in? Or your work place doesn't provide plugs. I do of course understand that only a very small percentage of North Americans live in a climate that cold and shouldn't be a factor in bringing a diesel over here.
I vote nay on the diesel.