We stayed at the Marriott Disneyland Paris 2 years ago, so I will put my 2 cents in. It is true that resort is a train ride to the City of Paris, but the train is not so bad. One day, we figured out how to get to Versailles entirely by train. We were able to stay late in Paris and take the train back. In my opinion, the biggest time waster was renting a car, although the car did allow us to take a day's drive from the resort and visit Rheims and the famous cathedral there and drive through the champagne region of France which is very picturesque. On our last day, we had to return the car, and spent the better part of a day driving to Paris, getting in some horrible traffic and trying to locate the rental car agency's spaces inside the Gare due Nord which was very frustrating and confusing. We mostly used the car to get to and from the nearby train station, which required parking fees in addition to the rental cost. On our last day, I noticed that the resort offered a shuttle service to and from the train station at a nominal cost. How I wish I had known that -- we would have willingly paid for the resort to shuttle us over to the train station.
Hope this helps someone in their planning.
Sally