zidane - iniesta is an interesting question. Personally, i think there is a certain level of world class, once a player goes beyond that, any comparison really boils down to personal preferences and statistics. And for me, both zidane and iniesta are past that level, in terms of talent, class, big game play, impact on the rest of the team, ability to influence or decide a game on their own, success on club level and international, etc.
Of course, there are still certain aspects where one is clearly better than the other, after all players are not identical. But compared to the overall amount of class these players have, it doesn't really put one ahead of the other.