Brother, you cannot compare a santro to vitz! seriously, thats an embarassment for the Vitz.
As far as your stress on petrol/gas affordability, I have a vitz 06, gives me 11.5 km/l MAX, and oh yes I drive with a heavy foot, in karachi traffic. . Also my daily running is abt 50-60 km in this car only.
Coming to the point, yes it might be a bit heavy on your pocket if you compare it with the running cost of your coure, but then again.. both these cars are in totally different leagues.
If you want a manual CNG car go for city 06-07, if you want a less driven JDM auto on petrol, go for vitz 06.