Next month my wife and I will be driving from Rome to the Amalfi Coast, with just a few hours to devote to either Pompeii or Herculaneum, but not to both. Which would be a better experience?
You can't go wrong with either choice. I vote for Pompeii, hopefully others will give their preference. Seeing the casts of the bodies at Pompeii adds a dimension you don't see elsewhere.
I visited both a couple of years ago. Pompeii is large and very spread out. The walkways are originals for the most part so watch your footing. I don't think a few hours will do it justice. I spent over 4 hours there.
Herculaneum on the other hand is much more compact with tourist friendly paths. A few hours will definitely give you a good feel for the cataclysm.
In addition just outside the train station for Herculaneum you will find minivans that will take you up to Vesuvius for a reasonable charge.