you only live once..
We are talking about buying a diesel BMW not taking a career break to trek the Amazon rainforest.
Though its the most silly phrase ever, you only live once but you live for a very long time in which you can buy many cars.
Personally if the running costs are not an issue or you can have a decent reserve fund then... buy a 330i instead. If low cost running is your thing, buy neither. The older they get the less sense the diesels make.