I've not been in Italy before but in my opinion, it depends on how convenient you want. The age of persons amongst your group.

If u have knowledge about the route or direction in these cities, you can rent a car. But if not, the train is a good choice. There are 52 trains a day from Rome to Florence and it takes only 1 Hr & 35 Min by the Euro Star. The cost around $45/person. You may calculate the cost between renting a car and taking the train again.