[kaisen]

The broader brush here is this:

How much money would you have to "save" in fuel costs before it's worth driving something you don't love?

Some people (lots of Pelicans, for example) like the sound, feel, torque, and smugness that driving a diesel gives them. They can justify it however the want, but when it comes down to it, they actually like it. It makes them happy.

Will your wife be happier driving a diesel? Worrying about adding BlueTec (urea) fluid? Dispensing diesel from smelly dirty pumps? Happy driving into the gas station seeing the sign say $3.179 for gasoline and $3.619 for diesel (it bothers some people)?

Will she be happier driving a hybrid? Being able to show her dedication and commitment to the environment? Would she get a smile silently pulling away from a stop under pure electric power? Watching the eco readouts and playing with her driving style to get the very best fuel economy numers? Some people love that about their hybrids.

Will she be happier driving the nicest car overall, without considering which drivetrain?

She's going to spend a couple hours a day in the thing.... she's got to like it, however she measures that. For some it's the ride/quiet/smooth characteristics. For others it's safety/size (lots of accident exposure). For others it's the technology/toys/sound/nav. Or comfy seats. Or, or, or... Only she'll know