I'd say Pompeii, if only for the reason that the excavations there have uncovered far more than Herculaneum. I was there about thirty years ago and I imagine more has been uncovered since my visit. Visiting Pompeii will give you a much better overview of Roman life in that time. However if time is really short, Herculaneum would, I think be the better bet as it is a more compact area.

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There is no "snack bar" on the site, even though it is listed on the map. Bring your own lunch and a water bottle. The site was the highlight of our trip to Italy.

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"The coverage of Herculaneum is pretty good - some more photos of the Theatre, Basilica and the Villa of the Papyri would be nice."

I agree - but they're hard to come by - meantime, the Herculaneum Conservation Project website covers the Theatre (see links). We'll keep looking though.

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