Unless you do between 15-20k miles per year there isn't any point in getting a diesel, the exception being the vehicle in question is a big and heavy one like a SUV or MPV.
There was a study done a few years ago, if you buy a VW Polo and only do 12k per year it would take you 14 years if you bought the 1.4TDI version over a 1.2l 12v petrol version. Takes into account the higher cost to buy the car and also the price difference between petrol and diesel.
Personally if I were to buy a smallish car it would almost always be a petrol version though not necessarily the smallest engine size available. In such size of a car it is unlikely you'd feel any difference as the torque on the smaller diesels aren't that fantastic. Recently there was a publication in Norway that compared the Ford Fiesta and whether the numbers actually added up in the real world, they used a 1.25 petrol 60ps, 1.0 EcoBoost 80ps and the 1.5tdci 75ps, the assumption was you did 20000km per year (average in Norway), they actually found out that it would take 13 years of motoring to break even when you buy the 1.5tdci over the 1.25l version and surprise surprise you would never break even with the 1.0l version as it actually consumed more fuel than the 1.25l.... not surprising then that Ford stopped selling the 1.25l in Norway soon after that article was published.