The other side of the coin is, of course, that you cannot budget to repair the diesel car (as all your money has gone on the initial outlay), so why should you budget for repairing the petrol car or assist with the running costs. You may, in certain scenarios, be better off using the money you've saved by buying the petrol car to fund your living costs.
Big problem with this is that you will find it difficult to afford repairs should the worst happen. You could find a slightly older diesel costing the same amount as the petrol car and then have the same contingency for both motors... then you'd have to factor in the costs of diesel parts versus petrol parts and general reliability