Another option that I just came across is the Sit Bus Shuttle, from Termini to the port.  It drops off at the port shuttle bus station, so you'd need to transfer your luggage from one bus to the other, but it's easier than the train and you're guaranteed a seat.  It's also three times the cost, but still pretty inexpensive at 15 euro.   Sit Bus Shuttle, servizio bus navetta tra Roma e aeroporti