Capri or Herculaneum?

I found Herculaneum somewhat less interesting than Pompeii. I'm confident I'd find Capri more interesting than Positano, so--setting aside logistical complications, which is risky when you're on a cruise--I'd definitely choose Capri.

But how did you feel about Pompeii and Positano?

I too enjoyed Pompeii more than Herculaneum. That's not a knock against Herculaneum though. However I enjoyed Capri significantly more than anywhere I visited on the Amalfi Coast. My advice is to study Capri and have a plan to stay a step ahead of the crowds because it will be busy if you stay on the main streets of Capri Town and Anacapri.

Have you spent any time in Naples? What's the date?

Hi Ks,
Below is our recent-ish TR about the AC. It includes a section on Herculaneum (plus a funny story based there). I go by 'zebec' over on Fodors. That TR is almost as long as the coast itself.
Have you considered either Procida or Ischia? Both are featured in part two of the above report. That part two is titled: 'Beyond Capri: an Ischia TR'.
cheers
https://www.fodors.com/community/europe/costa-damalfi-a-tr-1719110/page2/#post17517628

We’re all different… i love Pompeii but i love Herculaneum more. It’s much more intact making it far more fascinating to me. I’ve been to Capri many times and it doesn’t do anything for me, i find it boring. I’d stay in Naples and enjoy the city if it were me.

I would agree that a day spent in Naples would be something different for you since Capri will be crowded and somewhat similar to Positano, and Herculaneum isn’t calling to you. Naples is a vibrant, fascinating city and there is much to see.

I think it depends on what you want to do? If you enjoyed Pompeii, you're likely to enjoy Herculaneum; it'll be a similar experience. Capri would be a different experience but maybe not one you'd enjoy unless you're willing to head off to some of the scenic corners on foot. You don't say when your trip will be but if it's during high season, the piazza in Capri will be very busy, and same with the piazza in Anacapri. We escaped some of the worst of the day-tripping crowds by hiking up to Villa Jovis (generally paved path), to Natural Arch and the Pizzolungo coastal path, and exploring the villages early in morning before the ferries arrived (we were staying on the island for a couple of nights.) We skipped the Blue Grotto - never been sorry that - the high-end shopping and all of the guided tours. I do recommend the chairlift to the top of Mt Solaro for the view; the view from Villa Jovis is pretty sweet too.

You can also visit the churches and the Charterhouse of San Giacomo, and some folks have enjoyed Villa San Michele, which we didn't do. Lots of ideas here:
https://www.capri.com

I trust you understand that you cannot dare to do Capri independently from a Naples cruise stop. You must use the proprietary excursion from the ship, because the risk of missing the ship's departure is too great.

The two destinations are not comparable in any way. If you have been to Pompeii and don't think you missed something, don't go to Herculaneum. I would vote for Capri, but you did not tell us how many hours you will have on the ground from ARRIVAL on Capri to REBOARDING the ferry that takes you back to Naples (perhaps an hour ride? I went from Sorrento, which is closer.) Capri is absolutely huge, hilly, and largely pedestrianized. Four or five hours would only be a "taste" of Capri. But if the weather is good (I know, you have to sign up for the excursion in advance), it's a great day out. Have you looked at Capri on Google Maps to see how big it is?

Note that the ferry may have little fresh air or deck seating. Gulf swells could be rough or calm.