In Paris we are going to visit: Louvre, Rodin, Hotel Marine, Luxembourg Gardens, Montmartre, Orsay, Dali, Dior, Place des Vosges, Marmottan Monet, Tuileries, L'Orangerie.
We do not want to stay in Paris but do day trips either from Rambouillet > Montparnasse, or, our CDG airport hotel > Nord. Which train station would be better for our trips into Paris given the sights we will visit?
Montparnasse is slightly more central but there isn't much in it. Rambouillet would be nicer to stay in than the area around CDG.
Timewise I think it doesn't really matter. From CDG is Chatelet/Les Halles the most central located station for taking the metro to the mentioned places. But once you are used zipping around with the metro they are also from Montparrnasse easy to reach. Agree that Rambouillet is more pleasant to stay, would be my choise too.
Best is to study the metro / RER (train network) map before leaving. https://www.ratp.fr/en/plans
You do not choose the train station, unless you are talking about métro/RER stops.
Trains from Rambouillet go through Gare Montparnasse.
The RER B from CDG stops in Paris at Gare du Nord, Châtelet/les Halls, St Michael, Luxembourg, Denfert Rochereau, and Cité Universitaire. Corresponding to your itinerary, Châtelet/les Halls is likely the only stop you need, maybe St Michael depending upon your specific destination.
From either of these Paris entry points, you will likely want to connect via the métro to specific addresses.
You are going to be spending a notable amount of time commuting if Paris is your primary destination.
Most of the places that you want to visit are relatively easily reached from the RER B line from the airport because you can exit the train at Gare du Nord, Chatelet-Les-Halles, Saint-Michel Notre-Dame, or Luxembourg but staying near CDG is pretty charmless so I agree with Phillip about staying in Rambouillet and coming into Gare Montparnasse.
@ Tocard: I think the OP knows they can’t choose their train station but are trying to figure out which train station is better located for getting to their target locations in Paris. Then they will decide whether to stay at CDG or in Rambouillet.