New diesel engines and technology is really good. On par if not ahead of petrol technology.
In terms of economy as per UK
1 - lpg
2 - diesel
3 - Petrol
But i prefer diesel as no loss in Power as compared to lpg.
Especially with incentives offered here in uk. Diesel cars do 63 mpg with £0-£35 road tax.
Diesels are cleaner cars in terms of emissions omitted.