It depends how many miles your doing each year.

If you change yours for a diesel then you'll probably end up paying more for the car than the petrol equivalent and more for the price of fuel ( that applies in the UK but not sure where you are) and more when it does go wrong on pumps, injectors, etc.

Me personally, if I did less than 10k miles a year, I would go for a petrol, especially when petrol is cheaper than derv. Maybe keep what you have or get a newer 1.8/2.0 petrol model and treat it to some nice wheels and stereo.