Iniesta is without a doubt the more on form player. In terms of natural ability, i feel it is much closer. I guess you could create a case for either.
Kaká is of course much more athletic, yet they both have great acceleration with the ball.
The problem is kaká has been a part of an ageing milan team, and a brazil team that has seemed to lack leadership for a while. Having said that, i feel that if you put kaká in any team, he will play a crucial part to securing a victory, but he will need his teammates to play well and feed off his balls etc. However with iniesta there is just something about him whereby the whole team can play badly but he will still do things that will win a match.