Thanks so much for replying. Yes, there is so very much we are not doing and seeing and experiencing, and every time I have to make a decision as to what to miss, I suffer!
And I now realise this question has also been debated on the Ercolano forum. No easy answers there either.
I would go to Herculaneum which I found much more interesting than its more famous counterpart , there is also Oplontis which has very few visitors , its no comparison for scale think football to pea , but again interesting and without the crowds but its an individual property not a town. Given your time limit though Herculaneum would be the one .
My first visit to Pompeii was on an organised tour (accompanied by a Classics professor) and we were taken to Herculaneum too, which I found I much preferred. I thought it was fascinating to see the modern city rising above it. And it is much smaller, as already mentioned.
The second time I visited Pompeii we were also taken to Oplontis, and that was a real highlight. That second visit was a day-trip from Rome, with a school group (and an archeaologist explaining stuff to us....she was the one who added Oplontis to the day). The lead teacher had insisted on visiting Pompeii, although Ostia Antica, the port of ancient Rome, would have been so much simpler to visit. He said he didn't want to be "fobbed off" with just Ostia Antica. But we have been there since and it is wonderful, a huge site, with far fewer visitors than Pompeii gets, so more atmospheric. If you have time while you are in Rome you might go there for half a dayEdited: 4:41 am, June 10, 2018
How much time do you have? Herculaneum is a smaller site so if you are pressed for time you may want to consider it over Pompeii. Also, are you easily affected by sun and heat? If so Herculaneum has more shady spots to take a breather. Often people feel Herculaneum is better preserved but I actually found both sites to be. Both are fascinating.
Pompeii can be rather crowded in the morning, so as it is not very pleasant to visit some houses. If your visit has to be in the morning, I'd probably opt for Herculaneum.
Also, since time seems to be a constraint: Herculaneum can be visited completely in 2-3 hours, whereas you'd need 4 hours or more for Pompeii. Pompeii is huge, so that includes a lot of walking.Edited: 6:50 am, June 10, 2018