Herculaneum > Pompeii
That’s the most important piece of information you need to know.
Though both Pompeii and Herculaneum were buried by the eruption of Vesuvius in 79 AD, they were buried in different ways. Herculaneum was buried more quickly and at a higher temperature. Therefore – don’t ask me about the science; I’m just parroting what I read – the second stories of buildings as well as original wood was preserved rather than knocked over or burnt.
Pompeii is massive. You could spend 4 hours there and not see everything. However, many of its mosaics and artifacts have been carted off to museums, especially one in nearby Naples. Plus, since the roofs, second stories, and much of the walls of the buildings are missing, it’s quite difficult to imagine the city as anything more than ruins. You need a reconstructed picture, not just the descriptions of a guidebook or audio guide.
Herculaneum, on the other hand, is basically three streets by two streets. The buildings are frequently whole with the walls, second stories, and art intact. It’s very easy to picture living in the city. Since the size is more manageable, you can thoroughly explore the entire town within 2 and a half hours, if that. I much prefer it.
Of course, you can’t not visit Pompeii, but you just shouldn’t skip Herculaneum either.