Pompeii is such a major tourist attraction that nearby Herculaneum remains in its shadow. It is true Pompeii is much larger and in some respects better preserved (wooden furniture and mummies for instance were only found there), however, you have to pay for that by going along the often overcrowded lanes and missing so many houses that are now locked up due to the big crowds.
Herculaneum was destroyed at the same time and in the same manner as Pompeii. It is smaller but features similar houses with beautiful gardens and frescoes. Actually Herculaneum features some multi-storey houses, absent in Pompeii.
During my visit in 2007 almost all houses in Herculaneum were open. This gives you the impression you can really wander freely around the town and go wherever you like on your way to explore it. This was an exciting experience. There also significantly less visitors around.
Try to visit both cities if you can, but if you come during the rush season I would almost recommend to visit Herculaneum instead of Pompeii. It is beautiful.
If you go to Herculaneum try to visit the nearby Villa Campolieto, a Neoclassical mansion of the 18th Century belonging to the Golden Mile of the Vesuvian Villas.