Having just returned from the area, I would suggest Pompeii over Paestum. There is more to see there, and guides are more readily available. (Expect to pay about 10 Eur per person for the guide, plus the 11 Eur entrance fee.)
A number of the walls and frescoes from Pompeii are actually on display in the Naples museum, so that they aren't worn down by the elements. You might want to see them first.
A 2 hour tour just scratches the surface. You can stay and poke around by yourself after the tour is completed.
Don't get me wrong, Paestum is interesting, and worth a trip if you have the time. But if you can do only one, I say it's Pompeii.