I think it is a poor idea to stay on an island to visit distant land monuments. But that is an opinion. In addition to the excellent frequency of ferries, you might want to check the sailing times - for example, Capri is farther from Naples than it is from Sorrento. Others have noted that ferries can be cancelled, regularly for stormy water conditions.
Sorrento is probably the more central location than Naples, and it has quite a lot of ferries. It is a task to inventory all the ferry companies using just the internet. (And at the marina in Sorrento, no window for one ferry will tell you that the schedule of another company around the corner might better suit your schedule.) I suggest that you reckon with just how far, and how many modes of transport are necessary to get to Paestum. (Taking four public transit vehicles could add 2 hours of just standing around to the day. That's taking 1/2-hourly service as producing 15 minutes of latency on average-worse things happen in public transit, however-like lines of over 50 people for the next bus. Maybe it's not so bad in October.) Even with five nights in Sorrento, we had to cross off Paestum. We also decided to skip the Blue Grotto while in Capri for the day (on two different trips ... ... )
As Rick points out in his books, Herculaneum is not better because it's smaller. It's smaller because less of the new city of Ercolano has been torn down to EXCAVATE the ancient city. You have no idea what you might be missing, and what might be never excavated. Rick observes that excavated worship shrines in Pompeii make it certain that Pompeii had a Synagogue - but it has not been found. Herculaneum is important for specific buildings (some of which may be closed during your visit, like at all Italian monuments ... ... ) and for the different nature of the volcanic destruction. Herculaneum is also a long walk downhill from the Circumvesuviana station. Pompeii is "right there." Beware of visiting Pompeii from the Salerno train line - the station is not anything like adjacent to the excavated ancient site.
We enjoyed Pompeii, Herculaneum, and Oplontis, all nice in their own, and very different ways. But that is way too many ancient sites for the average tourist and his/her available time. (There is currently a touring museum show on the Villas of Oplontis, in United States venues.) Other factors, like weather, heat, and dining (there is no food inside Herculaneum, and you cannot exit and re-enter) are also important. I recommend Oplontis for those with modest but important mobility limitations.
Edit: Sorrento was the best base for OUR particular sightseeing needs. We slept one night in Naples, partly because of air arrival time. But we could have spent more time there, and I would have liked to see the palace in Caserta. (I think it's a choice between Paestum and Caserta, which depends on art history and European history interests of the travelers. But we got to neither.)
Edit 2: It's more than 4 public transit vehicles for Paestum from Capri: Although the Circumvesuviana station (i.e. bus to Amalfi, where you change for the bus to Salerno) is technically walkable from the Sorrento ferry marina, it's not practical to walk on your time schedule, or the need for energy for a long day to Paestum. I don't know how far it is from the Salerno bus stop to the train station. It becomes a nutty scheme. I have heard RUMORS of a ferry from Salerno to Capri, but in October? Not so likely.
