The Statement that Diesel Cars are better for emissions is a matter for conjecture, the Nitrous Oxide (Nox) and PM10 particulates are way too high, even the Euro 5 diesels emit two and a half time the Nox levels achieved by Petrol Cars from a long way back. Euro 4 Nox was double the Euro 5 figures.
Another issue is the relative scarcity of Diesel Fuel, refining Crude Oil into Diesel yields only 20% of the Barrel, Petrol gives 40%. If too many choose Diesel, how long before the Wells run dry?
The modern Diesel is too complicated, Turbo-Chargers/Intercoolers Adblue/ Eolysis additives etc, and then there is the issue of the Diesel Particulate Filter in the Exhaust, try googling that phrase and see how much trouble they are prooving to be.