INI suits a southern itinerary that starts and ends near Niš. Compare it with BEG only after including the onward and final drives; it is not a Belgrade airport.
Niš Airport lists rental providers and branch contacts. Arrange collection against the real flight arrival, ask whether handover is at a desk or a meeting point, and confirm the return procedure for an early departure.
A car is useful for rural stays, monasteries and regional journeys beyond Belgrade and Novi Sad. For city-only sightseeing, compare public transport and collect the rental when leaving the city. Include overnight parking and the trip back to the collection branch in that decision.
Compare airport collection with a later collection if you start with a city stay. Include the transport to a different rental branch and any extra parking nights when comparing the total cost.