In all honesty you would need to do your sums on this aspect.
You need to consider the whole picture
Will a diesel car cost me more to buy
Will a diesel car hold its value better
Which car will do the best mpg or cost per mile
What are the tax and insurance implications
What are the likely maintenance costs - diesels tend to be a bit more expensive at service time, etc
The best way is to set up a spreadsheet and calculate the overall cost per mile of running as fuel isn't the only cost of running a car and the fewer miles you do per year the fuel cost in some aspects can be the least of your expenses.
I travel about 2,000 miles per year mostly on short runs and I reckon my annual fuel bill at around £250 per year yet I spend about £350 on tax and insurance plus I reckon my car will depreciate in value by about £400. Without maintenace that works out at 50 pence per mile.