Messi rates Iniesta as the greatest all-round footballer amongst his team mates. Pep Guardiola, speaking near the end of his career in 1999 said, "People keep saying Xavi will retire me from professional football, but the truth is Andres Iniesta will surpass us both!"
So in my view, while Xavi is a superb player and integral to the team and the style it developed over the past 10 years, he does not quite match the multi-dimensional qualities that Iniesta shows all the time. But this is a bit of a silly battle anyway; they both fulfil different roles and are legends of the game in their own rights.