I've seen both and I generally prefer the dance before the dinner.
That way, as the guests finish dinner, they can feel free to get up and start dancing if they choose. It just helps with the flow and keeps the party from dying. A lot of times, the bride and groom don't get to eat quickly because people are talking to them or they have stuff to do and the guests are just stuck sitting around if the dancing hasn't been opened by the couple yet.