As per your question , you are going to travel around 821 km per month. As your running is in the range of 800 to 1000 km per month, the petrol version will suit you more.
where diesel engine provides good fuel economy, it is expensive to buy as compare to the petrol engine. On the other hand Petrol engines are cheaper, provides smoother, low maintenance and more powerful drive as compare to the diesel engines.
So in my opinion, you should go for petrol engines, and enjoy hassle free and powerful drive.
Maruti has a good and vast service network and there will be no problem of maintenance and replacement of parts at later stage but the resale value of the car might decrease as the company has decided to discontinue the vehicle. Despite this, there will never be any shortage of parts as manufacturers make parts for next 10 years. Also, as Ritz share a good number of parts with its sibling Swift there will be no shortage whatsoever. If you would like to consider, the Indian auto market has many other offerings in the same segment such as Tata Bolt, Nissan Micra, Ford Figo, Honda Brio and Grand i10, Maruti Swift, Chevrolet Sail hatchback.