Spain's dominance of possession and the opponent is down to Xavi and Iniesta and the play they bring with them. The other players on the team simply do not dominate teams the same way. They can all pass, shoot, create - but the flow and direction that Spain's game gets is largely down to the duo.
On the other hand, Xavi and Iniesta can so effectively outplay the opposition only because the other players on the team are so talented in different ways. Just look at Spain, it is a dream team. Casillas - possibly the best goalie in Spanish history; Ramos, Pique, Puyol - world-class "best in the world level" CBs; Busquets - one of the most astute DMs ever; Villa - best Spanish striker of the past decade; Alonso - one of the best passers in a long time.
Zidane's France had equally talented players but had a different playing style that didn't require the kind of innate understanding that you see between the Spanish players. So, in that sense, I don't see an advantage for either player (Iniesta or Zidane).
Like PearlBlack said, a lot of things have to go right for a team to be truly historic like Spain/Barca. When comparing individual players though, for the vast majority, Zidane will be remembered as the better player compared to Iniesta because his team's game emphasised individual brilliance more than Barca does.
Iniesta's dribbling skills are almost never meant to directly contribute to a goal. What he does is beat/draw in defenders to make space for his teammates to run into and set up an attack. He never tries to run directly at goal and score. He isn't Messi and you shouldn't think they dribble under the same conditions or with the same intentions.
