I am arriving to Paris on Nov. 11 at 655am and leaving from Paris on the 18th around noon. Would it make sense to buy a Navigo Decouverte or Mobilis tickets? What would you recommend? I figured i would make a day trip to Versailles on the last full day there and just get a round trip ticket seperately for that one. Also would it better to take the Roissybus or RER to get into Paris since i am staying in the Opera area?
Since you land on Friday a Navigo would only be good from Monday onwards. Even still I think it would be worth the cost to get the Navigo for the following week for Paris to Versailles by train and Paris to CDG by Roissybus. Those two trips would be nearly 20€ and the cost of Navigo per week is 21.25€ (plus 5€ for the card).
For the first CDG to Paris by Roissybus, just buy single tickets for that ride and focus on walking during the weekend or buy Mobilis day tickets if you plan on making more than 4 one-way Metro journeys per day.