We have never been to Paris and are traveling there, arriving on November 11 at 9:30am local time - can anyone tell us if it's easier to take a cab (and estimated cost) or public transit (RER/Metro)? Thanks for your feedback.
How many people? Its 9.75 euro pp using the RER/Metro train system. Take the RER B to Denfert-Rocherau then change to the Metro No 4 or No 6 line. No 6 line gets you closer to the actual Montparnasse station. Taxi would run in the 60 euro per cab range.
It's easier to take a cab.
Another alternative is the Air France bus service (not just for Air France air passengers) which runs from CDG to Montparnasse calling only at Gare de Lyon. The fare for that is 17. 50 (or 16.60 if prebooked online) euro per person. These are Greyhound-type buses with luggage compartments, not city buses.
Another factor is that the Novotel is actually attached to the Gare Montparnasse Vaugirard station. This is a side station on the west side of the main station and toward the back. Either the Metro station and the bus drop off are about 1/4 mile away, so there is still some walking to be done. A taxi would drop you at the hotel entrance (be sure you specify to the driver).
Traffic into the city in a taxi should be lighter as it's a public holiday.
Figure €50-€65 euros or so for a cab ride that would be a lot less stressful than train or bus into the city and can put you on hotel's doorstop. If you're talking the cost spread among 2-3 people, it's not much more per person than other options and worth it, IMO.
Just make sure before you get in that you have enough euros to cover the trip, or that the cab driver has a device to read a magnetic strip credit card- many do but not all.