If you are finding more value in the Maruti Wagon R, you should go ahead with this, as in the end, that is what matters to you. However, do bear in mind that the boot of the Wagon R is completely filled with the large single CNG tank and the spare tyre, thus leaving practically no space for even small bags.
The Tata Tiago CNG, on the other hand, has the spare tyre underneath the car and comes with a dual CNG cylinder setup which sits below the boot floor, thus leaving you with a decent amount of luggage space.

