I travel everywhere in Europe by train. Especially in France (in my experience) the high speed train system is superb. I wouldn't even try to deal with airport hassles. You should get a sleeper or first class for overnight, but I would definitely suggest all trains over all planes -- unless there's an ocean in between :).
If for some reason you go by day, you'll get some amazing views of the Alps and the Italian lakes.
The high speed train from Paris to Marseilles is a wonderful adventure and from there another train to Rome. I love train travel in Europe almost as much as driving in Europe. So, if time is not a problem, the train is a way to see more of the county and it is worth the effort. You can spend the night on the Mediterranean and take another train the next day to Rome. Try looking at Europass for train info.
I would suggest not take the overnight train if you do no purchase a sleeper;
· Our cabin was completely full no room to turn, etc, with 6 people (four we did not know)
· the train ride was 12+ hours
· there was nothing to view or enjoy at night anyways
Take a flight, save 12+ hours and enjoy Paris,