Pompeii or Herculaneum...

There is no right answer, some like to squeeze in Herculaneum because it is smaller, some really want to see Pompeii just because of its significance. What month? Heat can be brutal, and I'd find either difficult with jetlag.
Many hire a private driver who will watch luggage in car, a guide is a separate thing--they can be hired on the spot at Pompeii, not sure about Herculaneum. Doing it by public transport is a bit more complicated--look at map to see the difference in Circumvesuviana line and Trenintalia line to Salerno, which is where you want to get for transfer to Positano.

worthwhile will depend on you. Your desires, your expectations, your "listening to a tour guide and walking around stamina", your available time you have.

Herculaneum is smaller, better for the short attention span. It is also a bit more "easy to be pictured" for me, as the ruins are less destroyed in some ways as it was ash vs lava for the destruction. I could do Herculaneum in about 2 hours or less on my own, so you could probably do a deep dive with a guide in that short time. Both the "normal" yet "slow trains" to Salerno that stop at both are farther away from the entrances to the ruins than the commuter train to Sorrento.

The "famous ruins" that come to mind when you picture it in your head are PROBABLY Pompeii. It is MUCH LARGER, with MUCH more walking. You could stay there all day and not get to everything. You could probably get a private tour guide to do an all day thing if you wanted.

In my mind, Pompeii logistics might be a bit easier because of luggage storage (I don't know if they have any storage at Herculaneum as I have never looked when I went), but I will defer to the people who really know all these things.

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

The logistics problem is crowding on the Circumvesuviana train, which does not go all the way to Positano anyway. If you choose to pay for a car service, a 2-4 hour wait at either site is even more expensive. Good luggage checking at both sites.

ooo I just realized something! You could do the Circumvesuviana TO the sites (since the stops are closer) and then you could take a taxi to the OTHER station to get on the slow train to Salerno afterward (those are not walks I would want to do with luggage), it will save a tad bit of hassle.

I'm not clear on whether you're thinking about doing this the day you land in Italy. I would never attempt that, though my attitude is based on my own (dreadful) difficulties with jetlag and sleep-deprivation after the overnight flight. If you don't suffer as I do (or if you're going to be in business-class, lie-flat seats), you might be OK, I guess.

Pompeii has what seemed to be the world's bumpiest cobblestones. It's quite hard to walk on those cobblestones. You need to watch where you're going and really concentrate in order not to turn an ankle. If the cobbles get too difficult you can walk on the sidewalk, but those sidewalks are often much, much higher than the street. When you need to hop up or down at an intersection or to avoid people blocking the sidewalk, it can be a challenge, especially if you're short.

In other words, Pompeii is a workout beyond the sheer size of the place. If your trip is in the summer, there will also be the (likely) heat and lack of shade to deal with.