I hope this isn't TOO dumb a question, but what are the differences between the metro and RER? I have been to Paris twice and am familiar with the metro, but have remained oblivious to the RER. This time I am arriving at CDG airport and heading for our hotel near Denfert Rochereau, so RER is on my radar! I realize the RER makes fewer stops, but are there other (e.g. physical) differences? Would the RER be easier than the metro for luggage transport? Thanks.