Well as it's said before Xavi makes a much better volante and Iniesta a much better enganche. Nobody on this planet can quite top Xavi's perfect passing accuracy and range, his game reading ability or his skill at adjusting tempo, but on the ball he isn't nearly as dangerous as Iniesta.
Yes, of course they have different roles, it's just that a quality team with a gameplan focused on getting to Xavi can noticeably reduce his impact, (not to the point where he isn't useful or anything, just so that it should be acknowledged) and it seems like that never happens to Iniesta. He has off days, as does anyone, but when he's playing well he never seems bothered by the opposing players trying to shut him down, whoever they happen to be.
Also, Xavi has never been peerless in terms of passing ability, he's certainly the best CM but not for his passing alone.