too many things to consider:
may be both cars have comparable milage but with cng it may cost less and as if it comes with factory fitted cng then it might also be a trouble free experience.
both cars have good shapes and its really a personal matter. May be you consider city as ahead of time shape..
May be local Liana continues with the image of low cost spares as compared to city
availability also matters. may be you can get city in half time or vice versa
try to figure out your preferences and then seek answer