Should I return my rental car to CDG or ORY?

I would return the car to CDG, which is more convenient to Rouen. I have returned cars to CDG 3 times and it is quite simple. Just follow the Rental Car Return signs that are in both French and English. Taxi to central Paris is 50€ to the Right Bank and 55€ to the Left Bank regardless of the number of people and bags.

Via Michelin actually shows a slightly faster drive to Orly than to CDG, a little under two hours vs. a little over. But I wouldn't base my choice on that, since minor variations in traffic might erase the difference. I'd pick CDG because on the map it looks like a simpler route, because the rental car return there is easy to find, and because Enterprise will charge you less.

You've researched the cost difference to get into the city, but you might also consider convenience re where you'll be staying, like left vs. right bank. If you can forego the taxi, the RER "B" from CDG is very easy. Its cost of course depends on how many of you there are.

Thanks for the help. I'll be staying on the left bank in the 6th at hotel Abel Ami. I'll have my Garmin loaded with European maps so I think I would be ok at either location. I don't yet know how much luggage I'll have but after a long day and having never been on the metro or RER I'll probably opt for a taxi.

Since there are 2 of you and you don't feel comfortable with the RER, then a cab is the way to go. If you did take the RER to St Michel/Notre Dame, you'd still be 1/2 mile from the Bel Ami and need to take a cab from there, 10 EUR on the high side, so that is 30 EUR anyway. Spend the extra 25 EUR, then learn how to ride the Metro and RER while you are in Paris. It fun!
Speaking of viamichelin, they show going to Orly as having 13.70 EUR of tolls. They recommend a non-motorway route to CDG with no tolls. So there is goes more than half the taxi savings of dropping it at Orly.

Thanks. It looks like CDG makes the most sense. I don't mind public transit. I grew up in NY and have been to London. I think if we don't have much luggage and get in early enough we may use the RER and Metro to the hotel. I guess that can be a last minute decision.

You know, you could probably return the car in Rouen and take the train to Paris.

Trains are pretty frequent.

Just a slight correction on the flat fees from either airport:
The flat fees are good for up to 4 passengers and their luggage.
If there are more than 4 passengers, each must pay 4 EU in addition to the flat fee. There are no fees for luggage.

It makes no sense to return a car to an airport because it will take you two hours to drive there from Rouen and another hour to get from the airport to the city. Then add in metro or taxi time. On the other hand, you could drop the car in Rouen, take the train to Paris in an hour and a half tops and get from Gare St. Lazare to your hotel by metro or taxi.

I agree it would be best to return in Rouen. Unfortunately the rental agencies close at 6. Also, I'd have to take a taxi from Enterprise to the train station. I don't think I'll be ready to return that early. I do appreciate all of the suggestions.

Thank you