@matigo For trips to the south of France, I almost always choose the train. The journey can be done (just) in a single day, or you can set off a bit later, get off-peak rail prices and spend the money you save on staying overnight in Paris. I generally do that on the outward journey and do the return in a single day.
For longer journeys, the convenience of getting there in one day usually wins out, and sometimes for France if money and/or time are particularly tight.