I emphatically agree with Kathy that if one has luggage it would be a mistake to take the RER.

The RER trains are regular commuter trains from the suburbs into Paris and are often extremely crowded. The RER system is not in any way similar (other than being a train) to dedicated city-to-airport trains such as the Heathrow Express or Narita Express. I have made many trips around the Paris area in RER trains but never with more than a briefcase (when working) or camera case (when playing), onto both of which I held tightly.

It is much more practical and safe to use either a taxi or one of the transfer services to get from CDG into the city. When calculating costs, bear in mind that if the shuttle bus does not go to your hotel, then you will have to take a taxi from the shuttle terminal to your hotel.