There are some other things to consider when comparing Air vs Train.
First, consider total cost. The cost of the ticket may be only about $40, but make sure all taxes and fees are included, this could easily add another $40 per ticket. The train price you find is typically all fees and taxes included. Easyjet for instance charges another $12 to process your reservation.
Baggage Fees: As with any airline, plan on paying to check a bag, especially since most Budgets do not allow your big carry on in the cabin. this will be about $15 per bag (Train has no baggage fees) Beware also that you need to stay with the weight limits, or those fees go up very steeply.
Other transport costs: The train will leave from a station a short Metro ride from where you will be in Paris, and arrive in the center of Rome. For the flight, you will need to get to Orly, then in Rome, from Ciampino to your hotel. Someone else may be able to give exact cost, but figure $10 per person to get to Orly and about $25 per person in Rome.
By my estimates, that puts the plane costs from $40 to just over $100 per person, still reasonable, and if you throw in the cost of a hotel, maybe a wash, just make sure you investigate the costs and weight limits.
That said, Easyjet, in my opinion, is one of the better budget carriers with fewer "gotchas" on fees and rules, and if I were making the trip, would lean that way. An overnight train is something to be experienced, once maybe, but figure on a restless night rather than the Orient Express.