We did a similar trip last year, though with a few more days we stayed in both Naples and Sorrento. One note first: if you plan to do Pompeii and Herculaneum, a visit to the National Archeological Museum in Naples is strongly recommended. Most of the best mosaics and artwork have been relocated there, so it really adds to the experience.
If I had to choose a city to take a day from between Venice, Florence and Rome, Florence would be the choice. Not an easy choice, mind you. Venice needs time to take in, especially to enjoy the best times of morning and evening/night when the cruise crowds are gone, and Rome is endless. You could take months there and still be finding more to do.
We only stayed in Naples for two days, and Sorrento for three. Each had their positives, though I think a lot depends on how comfortable you are with urban congestion and less polished cities. We came to Naples first after Florence, and it was quite a culture shift! After the squeaky clean, spacious, pedestrian-friendly streets of Florence, the cramped chaos of Naples took a bit of getting used to. Sorrento, by comparison, was peaceful and beautiful, but also felt a bit bland after the intensity of the big city.
Both offer easy access to the archeological sites of Herculaneum and Pompeii via the Circumvesuviana train, though we found the area around the Neapolitan end to be definitely in one of the scruffier parts of town. We did those sights from Sorrento, so the train station was an easy walk from our accommodation, but I'm not sure if you want to stay close to the station in Naples. Maybe others can comment on that, but it may mean a taxi or bus ride each way.
(One word of warning about the Circumvesuviana: it is as full of pickpockets as advertised and then some. We saw very little of that elsewhere (even in Rome), but it's definitely a magnet for opportunistic non-violent thieves. It can also be quite crowded, depending on the time of day, as it's mostly used as a commuter train for the locals. This means that traveling with luggage can be a struggle, as managing to keep a hand on things in the crush of people is challenging. Definitely not impossible, especially if you pack as lightly as recommended, but stressful for some. This can be made much simpler if you can travel with luggage early on weekends, as the train is all but deserted then.)
Choose Naples if: You like adventure, aren't bothered by traffic and crowds, want a less touristy experience, and love food (oh, man, the food: pizza, baba, sfogliatelle, seafood, the list goes on - best food we had in a country full of amazing food).
Choose Sorrento if: you like peace and quiet, prefer views over urban intensity, want better access to the rest of the Amalfi Coast, and are nervous about theft.
If it means anything, the biggest regret of our trip is that we didn't spend enough time in Naples. It's definitely an intimidating place if you're not used to that level of urban crush (for comparison, we found Rome quite relaxed and calm), but once you get used to the rhythm of the traffic and the closeness of the streets, it is a city full of depth and surprises. We stayed near the Spaccanapoli, and walked pretty much everywhere (including the waterfront and the Archeological Museum). Of all the places we went, we felt like we missed the most in Naples.
