If you are going to take a taxi or rent a car, with normal traffic, the drive from Fiumicino will take from 35 to 45 minutes, while from Ciampino it usually takes at least half an hour because of heavy traffic. In this case, Fiumicino is closer and it takes less time to reach city center (Pantheon area). There are fixed costs for taxi from/to both airports.
Moreover, it is convenient to get to Fiumicino by public transport because there is a train station at the airport with two different train routes.
Thus, you can easily reach city center within 30 minutes. The price for the train varies from 8 to 14 euro, but you can also use more budget option and take a bus that goes to Termini central station for 5 euro. However, its not that difficult to reach the city center from Ciampino because you can take either one of buses that go to Termini station for about 5 euro or to take a train from the station located on a five-minute bus ride away. Prices for the Ciampino train are cheaper than in Fiumicino, single trip costs only 1.5 euro. It will also take around 30 minutes to reach the city center.