I always find it hard to answer these kinds of questions when you have to choose between two top players. If you had to organize a 1v1 at the top of their careers, Zidane would probably score more goals and have a more refined touch of the ball; possibly also make more unexpected moves. But imo it's a bit pointless to consider players as if it were an individual sport instead of a team one. Zidane was a great individual player, he could be the spark in a team lacking inventive. Iniesta was the perfect unit in a neural network. Would Zidane adapt as well to such a defined system? It didn't work as well as expected for Ibrahimovic, who is a top individual player as well. What I'm trying (and maybe failing) to say, is that Zidane would be the better of the two in an individual sport and would probably improve almost any team, but in football you should always consider how to make your team better, and I don't think Zidane (if he played in those years) would have been a better choice than Iniesta for Barça.