Diesel or petrol\CNG engine car?

we are planning to buy a car by this year end. There is a confusion over buying a diesel engine car and buying a petrol engine and fixing a CNG kit to it. Fitting a CNG kit to petrol engine car normally equals the cost range between them . Is it worth to kit a CNG to the car. We would use the car 30km (aprox) per week. What would be best for us? please help me out.!