I have taken the RER, taxi, and the bus. If money is not a consideration and you're elderly or have a young child, take the taxi. I normally took the RER when I was younger. So I did it again this past June with my toddler. There was some kind of train worker strike so we had to change train to get to the subway. Of course we changed to the wrong train (my French is pretty poor and the agents at the change stop "didn't" speak English) and had to retrace our steps. Needless to say, it was a bit stressful running around with a tired young child and luggage. There might have been steps in the train/subway stations but I don't recall it being too bad. On the way back, we just took one subway from the Marais district to Arc de Triomphe and took the bus. It was easy. Anyway, if you're physically fit, want to save some money, and can adjust to unexpected situations, take the RER or the AF bus. Don't worry about dragging luggage in the subway or on the street. You'll see plenty of people doing it.