The difference in fuel consumption between two cars that are identical other than their powertrains will depend entirely on the nature of how they are driven. In stop-start traffic and urban driving we would expect a hybrid to perform significantly better than a petrol car, because the frequent change in speed gives the opportunity for regenerative braking which will charge up the battery, and allow for electric-only driving. Conversely, higher-speed motorway work will give little or no advantage to the hybrid, where it cannot run on electric only and is potentially a heavier vehicle than the petrol only version. Which will deliver the best fuel consumption for you will depend on how the vehicle is driven, under what conditions and on what kind of roads.