I recently changed my car and I made the same consideration, before making my purchase.
Previous car - 1,600cc diesel, 5-speed manual. Average 45mpg, 3 years old.
Current car - 2,400cc petrol, 4-speed automatic. 30mpg, 3 months old.
You'd think it would be cut-and-dried wouldn't you?
However, based on my annual mileage (7,000) and the very important fact that diesel costs on average 55p per gallon MORE than petrol, my new car will only be around £3.50 per week more expensive to run, than the old one. Pretty good, considering it's almost a litre bigger in capacity and an auto as well
Of course, if I'd gone for a smaller capacity motor, I'd actually be financially BETTER off with a petrol engined car...