Not all diesels are equal, just like not all petrols are equal. So the question isn't as simple as that. Should you buy a diesel? If it's a good one yes. Despite the emissions stuff, the VW engines are brilliant diesels for long commutes. But if you drive around town, the Ford 1L Ecoboost engine is fantastic.
Best thing to do is use a calculator -
http://www.which.co.uk/reviews/cars/...sel-calculator
Punch in the details of your vehicle options and this will tell you what is cheaper.