seriously? we have 3 sites of posts and like 90% of the posts so far were like "itachi is 10x stronger" "he'd destroy him" and "he'd kill im in a second"? is nobody here basing his opinion on reasons?
momentarily i dont know whether sasuke could break tsukuyomi, the whole fight would be decided by that. apart from that the fight is pretty even.
taijutsu: sasuke is superior. itachi never showed even an ambition to fight someone in close combat, whereas sasuke already in the fight against him tried to get close to him. now he's probably even stronger and i doubt that itachi could beat him in hand to hand combat.
genjutsu: itachi's tsukuyomi is superior, but apart from that, i doubt that they are so differently strong. we have seen sasuke beating strong people with single genjutsus, i think he's capable of breaking any genjutsu apart from tsukuyomi.
ninjutsu: sasuke. raiton-mastery+amaterasu-shape beat exploding clone all day. you could argue that itachi's susano'o has yata and totsuka, but sasuke's has a bow, which should allow him to keep the distance until both are too exhausted to use it further.
tactics: itachi. sasuke can be cunning if he isnt currently braindead, but itachi is probably just overall cleverer.
so all in all it's pretty much a draw considering their abilities. sasuke is more powerful in reality and itachi would have to use genjutsu and intelligence in order to beat him before sasuke gets close enough.