I am confused between Maruti Suzuki Ritz and Wagon R CNG?

Go for Wagon R CNG.

The maintenance cost of a company fitted CNG car is equal to that of a petrol car. The petrol engine of the Ritz is more powerful than that of the Wagon R, but company has announced that they will stop the production of Ritz very soon as it did not sell in good numbers. You should go for Wagon R as it will provide a decent amount of power and space, with a nicely working CNG kit with 60 liters of capacity.