The better option out of the above cars is the celerio CNG as it is the VXi model in which the kit is fitted whereas in Wagon R, the CNG kit is installed in the LXi model. You will get more features in the celerio and also the design of celerio is better than that of Wagon R. No problem is faced in a company fitted CNG kit and they are certified by the company itself. Maintenance cost has a minor difference than that of a simply petrol vehicle and the amount is easily covered by the lower running cost of CNG vehicles.