With Eurail, the main benefits for me were the freedom to just up and go somewhere for cheap (after the initial payment) and the location of the stations. With the discount airliners the prices can be a steal on weekdays or in advance, but if you want to make changes it can be troublesome. Also, getting to/from the airport can add some cost and hassle. From Charles de Gaulle to the center of Paris by metro is 9.10 euro each way: oddly, if you go break up the trip from the airport to the nearest zone and then into Paris it only costs 7.80 or so. Most train stations are located in an easy location to navigate near the center of the city.