Amalfi Coast or Cinque Terre?

I like the CT option because it fits into the rest of your itinerary without needing to travel 3 hours out of your way. The coastline of the Amalfi Coast ( AC) is similar to CT but more crowded. if you’ve seen Big Sur, it won’t be as stunning to you as, let’s say, someone from Kansas or Nebraska who’s never seen Big Sur or even Laguna Beach, Ca.
The other reason for CT is you can start one day in a place like Vernazza and end it in a place like Santa Margherita Ligure or Rapallo- all while passing by Riomaggiore and Portofino! That is a lot of effortless sightseeing by train and boat packed into one fantastic day!
Finally, there will be 14 Cruise Ships docking in Naples from 9/4/25-9/7/25 with more docking at Salerno. You can bet there will be throngs of passengers daytripping to the AC!
Have a great trip!

Under the circumstances, I'd suggest the Cinque Terre this time, primarily for two reasons....

1. Budget - everything tends to be more expensive on the Amalfi Coast.
2. Travel times - getting to Positano especially is going to take longer. There's no rail service there so you'd have to travel to Naples first and then south to Positano, with the last segment being by bus, boat or car service.

In early September, both of those locations will likely still be crowded, especially if there are cruise ships in the area.

Monterosso is a good choice for a location to stay in the Cinque Terre. It's the largest of the five towns and has the best beaches of the five. Note that some sections of the beach are pay-per-use and some are free. Your hotel can tell you which areas to go to.

For the hike on the Sentiero Azzurro between Monterosso and Vernazza, I found that it's easier to start in Monterosso as there's a steep uphill section at the beginning and it's easier to do that going uphill rather than down. Note that a Park Pass is required to hike those trails.

Car rental..... note that each driver will require the compulsory International Driver's Permit for driving in Italy. These are used in conjunction with your home D.L. and can be easily obtained at any AAA office for a small fee. You'll also have to be vigilant to avoid the ZTL (limited traffic) zones that exist in many towns in Italy. Each pass through one will result in hefty fines which you likely won't know about until several months after you return home. Travel by train from La Spezia is not difficult, although there are usually at least two changes on that route. The rail station in Siena is at the bottom of the hill so travellers need to take a bus or taxi up to the town.

You might find it helpful to have a look at the Rick Steves Italy guidebook, as will provide all the specific detail to plan your trip. The new edition will be released in Summer 2025.

Having been to both (easy for me to say), I'd choose neither! Since you are open to renting a car, I would look at somewhere on the Tuscan or Ligurian coast that will not be so crowded (and would have much better beaches for swimming). But, if you have your heart set on one of these, I would definitely choose CT because it maximizes your time much more. Congrats!

Having been to neither I would choose neither. Having read every Italy guide book, including Rick Steves Italy and Rosie Whithouse's Bradt guide Liguria, from cover to cover over the last three years, repeatedly, it is very clear that the way to enjoy CT and Vernazza is by staying the night but never the day. Other charming Ligurian coastal villages such as Camogli, Levanto and Santa Maria Ligure seem more than satisfactory.

AC seems ritzy, glitzy and chi-chi (RS-speak for posh), which can interfere severly with the relaxation factor. The Gargano peninsula seems to be the new alternative Amalfi. Go west, to all parts of Liguria west of CT.

Dump Naples. The authorities are very clear the volcano will erupt again, on top of 1 million Neapolitans. They have been offered to relocate but apparently there are no takers (according to Insight Guide Italy). There has recently been a volcano hazard on the island of Vulcano too. Sooner or later!

My personal opinion is also that your Cinque Terre option is preferable given your itinerary that concentrates on the part of Italy north of Rome.

Although, if I could get rid of most of humanity and enjoy the places without the hordes of people who go to both destinations nowadays, I would probably like the Amalfi Coast better.

If you decide to avoid the Cinque Terre altogether, and look into the Tuscan coast instead, as some have suggested, then I'd rent a car in Rome and head north along the coast to the Maremma area. The portion of the coast on the Mt. Argentario Peninsula and the nearby islands of Giglio or Giannutri, is the best in Tuscany and would make a perfect seaside getaway for your 3 nights on the coast. You can visit Giglio island on a day trip from Porto Santo Stefano (1 hr by ferry)

https://www.tuttomaremma.com/en/
https://www.cnn.com/travel/article/monte-argentario-southern-tuscany/index.html
https://argentario.it/en/
https://www.visittuscany.com/en/towns-and-villages/monte-argentario/
https://www.giglioinfo.it/en/

The first night we ever spent in Europe was in the Hotel Baia in Monterosso for our 25th anniversary trip many years ago! The hotel was pretty basic, but fine then. I’ve been to the Amalfi Coast more recently. I enjoyed them both. As others note, the Cinque Terre fits better into your itinerary.. Congratulations and have a wonderful time.

Having been in the AC recently, May 2023, I would avoid it , due to the massive amounts of people there, even in the shoulder seasons. There is no fast nor efficient transportation mode. Being in crowds isn't relaxing. The beaches are made of stones. On a recent Facebook group post, a traveler showed the lines- and hour wait- at all the ACferry harbors, and this is September. This is an area that has been ruined by its Instagram success. And if you're day tripping, you're coming right into the thick of the masses.
Sorry for the downer, but that was our experience. Good luck planning. Happy travels.

Better Amalfi Coast option. We did this in 2022.

Take the train from Rome but get off in Salerno. Short walk to ferry. Stay in Minori or some other smaller town. You will save loads of cash and crowds are minimal. Our villa rental was affordable and had steps to a private platform on the water.

Splurge for a private boat tour along the coast. Positano looks better from the water anyway.

If there is anything you want to see, hire a private driver on your last day. You can be taken to the train station in Naples and pit stop along the way. We did this from Minori to Pompeii and then were dropped at the Napes train station. We used Joe Banana Limos.

We did Cinque Terre in 2010, so maybe our tips are outdated. But that area was also exceptional. We were staying in Volterra and did a day trip, leaving a car at a train station farther north and just taking the train in. Deciding between the two regions would be like choosing between my children, I can't do it.