The difference is that Modi takes ownership and responsibility for the Government's performance - bad or good.
Unlike the Gandhis who said they don't interfere and the Governments actions are separate while running them like puppets. When the Government did something wrong, they never owned up, when it did something right, they took credit.
Its exactly like the system of the British in Bengal in the early days of their guile. The nawab had all the responsibilities of the kingdom and no freedom but was the public face of the government. The british(EIC) had all the resources and power and no responsibility.
It makes a massive difference. Sonia was not accountable to the nation because she was not PM. She ran the Government from behind closed walls guided by a small coterie of people divorced from the ground reality.
The electorate is not stupid guynextdoor. They massively voted against a Government that was more accountable to Sonia than the public. Had Sonia been the PM, this rout might not have been as bad as it turned out to be for Congress.
Modi is the PM and therefore directly responsible. It makes a humungous difference.