When we rented a car in the UK, we were told to pick the diesel vehicle as it was great on mileage. OK. So it got pretty good mileage. But did you see the price of diesel? It was about 12p per liter more than unleaded fuel. That really adds up.
We were later informed that it would only make sense to get a diesel car if you were inclined to make heavy mileage, though, the exact figures were not forthcoming. However, recently, I saw an article about how long it would take to begin seeing a savings on a diesel car. First of all, diesel cars start out more expensive. So that needs to be worked into the cost. The article compared 4 different cars, in 4 different classes. The cheapest one took nearly a decade to turn over into a cost-savings investment. That is, if you are doing average mileage. It would be a bit sooner if you did excessive mileage, maybe a year or two sooner.
So, unless you own the vehicle for that long a period, it definitely is not worth getting a diesel car. And if you are going to rent a car for a week or two, it makes absolutely no sense to be persuaded to take a diesel vehicle.