We have been to both -- 5 hour personal guide tour -- starting with Herculaneum in the morning. They are very different. Herculaneum was mostly destroyed by the lava flows whereas Pompeii was destroyed by ash. In my opinion, Herculaneum is better preserved. Also, Herculaneum was a blue-collar city, whereas Pompeii was considered wealthy with many part-time residents from Roma during the summer months.
After touring both places, you must go to the National Archeological Museum in Napoli, where most of the original frescos and artifacts from Herculaneum and Pompeii are on display.
One difference is, the heat from pyroclastic flows preserved a lot of the wood (in carbonized form) in H, leaving doors that still swing on their hinges, upper storeys of buildings, etc. It’s probably a good thing that Ercolano is atop the ancient site, making it unlikely most of the ruins will ever be exposed to the open air, modern weather, and acid rain.