I would like to add to the comparison on which is better for the environment:
-diesel emits more toxic nitrogen compounds and particulate matter
-it emits lower amounts of carbon monoxide, hydrocarbons and carbon dioxide

Compared to petrol cars, diesel is usually more polluting because catalytic converters to lower CO2 produce even more nitrogen compounds, which are toxic for us. Of course, new systems and frequent maintenance makes a diesel engine similar to a petrol one when it comes to pollution.