One part of this consideration that for some strange reason never seems to come into the discussion is the cost of fuel. When I have rented autos (in the Netherlands), fuel, on average is twice what the rental price is.
Currently, in France, fuel is €1.66 per liter, or about €6.64 per gallon (
http://www.mytravelcost.com/France/gas-prices/). Driving in the summer heat and frequent stop and go will use more fuel than driving on a high-speed motorway.
IOW, even if you can find a way to save money on the rental, you won't be able to escape from the fuel pump and it will be quite expensive!