If you are staying walking distance from Termini station, the train is an option. €14 per person and 32 minutes journey on the Leonardo Express.
The taxi from anywhere in the city center takes about the same time. The cost is an all inclusive flat fare of 48€. You are going against the commute traffic, so no issues getting stuck. Even with traffic it’s a 45 min taxi ride.
If you are not staying near the station, then taxi plus train may be not much less than the 48€ taxi fare all the way to FCO, especially if there are two or more of you traveling together (2 train tickets are 28€) and/or with bulky luggage.