My average daily run is 50 Km / day...Do you suggest me to go for a petrol or diesel version of Skoda Fabia?

Go for the diesel, the 1.4 is a very efficient, long lasting engine. The 1.2 gas is the base in the Fabia, the 1.4 has better driving dynamics.

Agree with my predecessor. The diesel has the torque, lots of it, where & when u need it, at about 1900 rpm. That means u can (almost) forget shifting gears & take it nice & easy. The mileage of diesels is better than that of comparable gas engines.
The COST is a different story - with the surging fuel prices you'd better consider how many miles u get out of ur $ (or € or £.), not only in terms of fuel, but overall - depreciation, maintenance, tires.
To figure out which suits better ur typical driving pattern, u shoud find out the BEP (break-even point) for the two models.
50 km/day x 230 weekdays = 11,500 km/year - looks like a strong case for the gas engine - unless u let ur heart decide.