Naples to Florence in April: rent a car or take the train?

My husband and I will be arriving in Naples early on a Monday morning in mid-April (after taking an overnight ferry from Palermo). We are flying out of Venice on Friday morning. We would like to see Pompeii and Herculaneum on Monday before heading to Florence for Tuesday and Wednesday and then on to Venice on Thursday. Here's our dilemma: Do we rent a car after arriving from the ferry so that we can drive to Pompeii and Herculaneum and then to Florence OR do we take a train to see the sights, looping back to Naples and taking another train to Florence? We will have already spent the first part of our trip in Rome before heading to Sicily, so we don't need to return there. Please help!