Hi Amit,
In my opinion if you are buying a petrol car, I20 will be a white elephant.It was lowest mileage and bit underpowered. Otherwise it is very good car, can't compare the boday quality and the space with other cars. Ritz is a very good car with good mileage and cost much less than Swift or I20. The new Swift petrol is a very good car.
So in short if you want space,ride and comfort go for I20, mileage and price go for Ritz and if you want mileage,looks and bit of space go for Swift.
But in case of diseal the case is opposite, I20 is way ahead of the other two in terms of power,comfort,saftey,features,space.... lots more 
Cheers and best of luck,
PJS
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