Nissan March hands down, and why is that? It'll be a recent import instead of the Vitz which will be a pre-owned vehicle and used with God knows what type of care.
Now these answers are based upon my personal experiences, I have had a Vitz 1.0L in the past and I recently imported a March 1.3L.
I told you in your other thread that the average is great. A full tank can easily give you 700-750Kms on the highway and intercity average isn't bad at all either 665Kms+ is what you will be getting?
Maintenance isn't an issue for both vehicles. Your regular maintenance parts are the engine oil filter, air filter, cabin air filter. Your long-run maintenance parts would be spark plugs, brake fluid, brake pads, fuel filter/pump, gear oil/gear chamber and o-ring gaskets. These are available throughout the country, don't need to worry about that.
The march has an extremely comfortable drive quality, as compared to the rattling and less comfortable drive quality of the Vitz.
March has the better powerhouse obviously because of the 1240cc inline-4 cylinder 91HP engine. Breakdowns depend upon how you maintain your vehicle. If your vehicle is periodically maintained it won't breakdown on you.
Why will you need to install a CNG kit when you will be getting 650-700Kms in a full tank in intercity driving?