Granted it has been about 5 years now, but I know our train made a stop at Mantes La Jolie. In any case, you can easily get to the airport by train from the center of Paris, and it sure would seem like it would be lovely to have a last night in Paris vs. being at the airport the night before given your flight is in the afternoon. We have made a handful of trips now to France all ending in Paris. We enjoy that last evening at the end of the trip and then get up in the morning for one last pastry before heading to the airport. Logistically, its easy, but, like the commenter above implied, it depends on your energy level, and as was also stated, there really is no right or wrong answer, its a personal choice.
In terms of rental car return, our practice has always been to return our cars in a smaller city and then take the train into Paris, much like you've planned. Even with the airport on the far outskirts of Paris, we have had no desire to drive there. I would guess if you were to change your rental car contract at this point, the price could be considerably higher.