I'm going to break it down from a story telling point of view, what things could possibly happen and the story not fall apart right here and now.

Itatchi wins - Gets sasuke eyes goes after madara... Not likely, Sasuke being the main supporting character of this story, almost the main character at times like these. Sasuke losing his eyes here would hurt the story tremendously, and theres no way it could be sasuke's year, this takes away sasuke as being Naruto's rival, which is very important to the story.
Estimated probability = 3.2%

Itatchi Wins but is interrupted.. Who could possible interrupt them.. It seems Tobi/Madara is going to have his hands full with konoha.. Considering that he has quite a few Hokage Lvl Fighters to deal with, i believe he has his hands full. Naruto/Kakashi/and the Shodai remake/Sakaru healing.. No way he could own all of them so quickly.. no counting that kakashi has MS also... Zetsu is not interrupting anyone.. as far as we know he can't even fight. The only chance is Madara, but if you think about it.. Maybe Madara thinks he can bend sasuke more to his will and help him in his battle.. He knows that sasuke is probably coming after him next. I don't think he wants to break it up at all, he wants one of them to obtain the true power of the Mangekoyou Sharingan to aid him in some way..

Estimated probability = 13.6%

Itatchi wins - and his true feelings comes out for his brother and he can't take his eyes.. Considering The sentimental side that was shown of itachi not to long ago, this also has a chance of occuring but i not so likely considering the evil laugh/face look itachi has just shown of his true self.

Estimated Probability = 5.5%

Sasuke's chances have alot higher success rate of happening from a story point of view... Theres alot of ways you can continue the story if Sasuke won this battle.

Sasuke Wins - Sasuke wins but chooses not to take the path of Mangekoyou Sharingan and doens't take his brothers eyes right now. He lets Itatchi live with his defeat and him knowing that he cannot defeat his lil brother. Later on in the story itatchi sacrifices himself to save his little brother and gives him his eyes giving birth to a pure verison on the magenkoyou sharingan, thats not evil and has the power to defeat the dark sided verison which is powered by greed and hate.

Estimated Probability = 33.7%

Sasuke wins = takes itatchi eyes and uses this new found power to help him achieve his goals of killing both itatchi and madara.. This is not evil of him to do.. His clan got wiped out.. he wants revenge.. He is the avenger and hes willing to do whatever it takes to take his revenger including taking his brother's eyes. Sasuke then becomes among the top 3 strongest ninja in the manga series and then the year of sasuke begins.

Estimated Probability = 56.9%

Sasuke Wins = but is interupted by Naruto, and Naruto does all the little speech thing and try's to talk sasuke out of taking itatchi's eyes. This could happen cause it could easily be one of Naruto Bushin's that made it through to the hideout. Sasuke chooses to take itachi's eyes anywayz and uses the power on Naruto Bushin.. Or Naruto manages to talk him out of it and his brother escapes for now.

Estimated Probability = 24.3%