Both of these teams are better than you think.
Dallas? Don't let their overall record and point differential fool you. They have gone 64-16 with Dirk Nowitzki and Tyson Chandler in the lineup. They also won both regular season games against the Heat. Their ability to space the floor and pass also means they are one of the only teams in the league whose crunch time offense is something other than an ugly ISO into a packed lane.
Miami? After losing to Dallas on November 27th, they have gone 61-19. More impressively, they have gone 27-6 in their last 33 games, a 67 win pace, and looked better than Dallas in the conference finals. And Udonis Haslem, their 4th best player, is finally becoming healthy. And they have the best player in the series.
I think Miami is the favorite, but it should be an interesting series.