Pompeii vs Herculaneum

Ercalano is much smaller than Pompei, and in many ways more preserved. There are more buildings extant, and I think more approachable, more understandable. Easy train ride from Naples or Sorrento, then stroll downtown to the site.
Three hours, maybe four hours does the site justice.

I agree with Aussie for the same reasons, although they’re both great.
Herculaneum is so well preserved, you can still see tables and restaurants, and public toilets… can’t see any of that at Pompeii, but Pompeii is much more grand, much more expansive. Herculaneum is small, kind of in a canyon, felt a bit claustrophobic to me.

I visited both years apart, starting with Herculaneum, yet Herculaneum made the more lasting impression. It is amazingly well-preserved and not overwhelming. Also, if you are short on time, it is a half-day excursion from Naples instead of a full day.

https://community.ricksteves.com/travel-forum/italy/herculaneum-and-pompeii-questions

Edit: In addition to the comments in the linked thread, I'd point out that Herculaneum can have some overhead associated with it. It's a 10-15 minute walk (steep downhill) from the Circumvesuviana. It also has no restaurant, and you can't re-enter on the same ticket if you break up a visit with a meal. I only found a few restaurants on the main street in modern Herculaneum, but we got a good (near 2-hour, as usual in Italy, regular meal hours only) meal in a very modest place. I'd compare it to an American Formica-table pizza and pasta place, although of course, both were considerably better than in the U.S.

OTOH, Pompeii has a large, air-conditioned, glorified cafeteria, near the Forum, inside the paid site. It's pretty crowded, and there are lines for the bathrooms. But it is a retreat from the sun and heat. The food was okay, not as good as a restaurant. Pompeii is generally level, once you get in and very close to the Circumvesuviana station.

At both sites, there is always a risk that a certain Villa you want to see may be closed, either for conservation or a shortage of guards, or whatever. This is "less" true for Herculaneum, simply because it is so much smaller, and because so many of the buildings are open-sided, passive viewing from behind a railing. However, when we were at Herculaneum, some of the key interiors, down by the former waterfront, were long-closed. That was some years ago.

Although I had Rick's latest book in my hands, I found it hard to follow his walking tour of Pompeii. I think some of the routes had been closed off by the management to better control the sometimes long lines for the brothel. I mean that streets from which the brother wasn't even visible had been made one-way, or closed down entirely.

I would say "both", but if I had to choose one, I would take Pompeii, for the Amphitheater, Forum, and Villa of the Mysteries.

We allowed a whole day for Pompeii; and a second day for Herculaneum, plus Oplontis and Stabiae. If you have time, try to see Oplontis and Stabiae - very few, if any, other people and lots to see.

http://pompeiisites.org/en/oplontis/

http://pompeiisites.org/en/stabiae/

I would also maybe pick Herculaneum over Pompeii. It’s just a much more manageable site, whereas Pompeii can be really hot, crowded and tiring. Part of Pompeii’s appeal is it’s scale and grandness, but Herculaneum feels much more personal. The buildings are generally intact, with preserved wood, roofs and upper floors. You get a much clearer sense of what it was like, while Pompeii requires some interpretation. I’ll also note that Herculaneum is open quite late in season. This may impact your plans. It’s possible to see both in a day if you do it right.

Nothing much to add but we had some sandwiches, drinks and snacks at a local shop for lunch at Ercolano and it made for an enjoyable visit. For both we took the train from Naples.

We did both and they are both amazing, but Pompeii left a more lasting impression even though Herculaneum has a few murals and houses that are better preserved. Here's how to make the most of the visit to Pompeii:

1) Tour the Archaeology Museum in Naples. Rick has a free 1-hour walking tour that hits the highlights. You have to come to Naples anyway so you're already there, and the museum is 5 minute taxi ride from the train station. They had free storage lockers for bags and even our suitcases. All the murals and mosaics and other objects of interest that were found in Pompeii and Herculaneum were taken out and now are in the museum. See these first and you can imagine them in the ruins later. We then took a cab straight to the ferry dock and our hotel in much quieter and lovelier Sorrento, which is a convenient base for Pompeii.

2) Hire Antonio Somma, the personal tour guide Rick recommends in his book (pompeitour.com), to give your small group a genuine private tour. Start in the morning before the sun is too hot and before the crowds are a swarm. He took us away from everybody down the lonely streets, showed us plenty of interesting things, villas, shops, a bakery. When we did go to the brothel and other spots with a line and tour groups waiting, all the other guides knew him and he schmoozed with them and cut right in front each time, and nobody made a peep. We were there about 2 hours or so and felt fully satisfied we'd seen Pompeii. It was a lifetime bucket list for my wife so we weren't skimping or rushing.

We did Heraculaneum in the morning, lunch in the town (nothing special, good basic food at a reasonable price) then walked up and caught a (local) bus to Vesuvius with enough time to walk up to the rim, bus then train back to Naples and walked a block or two for a drink (Aperol spritzer) and snack outside. Pleasant day in October. We had done the A museum on the previous day (early morning when it opened) and saw the best of Pompeii murals and mosaics.