Went to Paris for a short break last year , and ended up using Line 6 and RER A to get to the Eiffel Tower
RER A has double deck trains , which are air-conditioned , but have 315 style 3+2 low backed seats on both decks , and can get very busy
When I went the entire centre section from La Defense to Nation was closed , so took Line 6 .
Would avoid Line 6 . Screechy , horrible , poorly maintained trainsets , although it did feel like it was in a time warp , rather like the Bakerloo Line . Had to cover my ears due to the wheel ( flanges I think ) between Nation and Bel Air , although the line is rather scenic between Bir Hakkeim and Trocadero
The stations did not feel as safe . There was a slightly threatening atmosphere , due to the dingy , wide empty corridors that we went through ( although some stations were nice , like la Defense ) , and there were far fewer staff around patrolling and on the platforms . Don't know if this is the same at peak times
Fare evasion seems to be high . When at Torcy station , I was buying tickets , when a train arrives . Half the people vault over the barriers , RATP staff just sit in their office .