Both have virtually the same set of jutsus, same eyes, but Madara has the much greater stamina and chakra reserves. In terms of raw power, Madara easily crushes Itachi- FACT.
Madara is an intelligent fighter (he must be right?), but so far, he's preferred to ovepower his opponents rather than outwit them. Against a skilled tactician like Itachi, this could very well result in his downfall.
BUT, from what we've seen, Itachi is a very perceptive and intelligent fighter, who can figure out his opponents weakness, whoever it may be. And throughout the series, we've seen intelligence triumph over power. Just look at Shikamaru and all his fights. Look at Itachi finding a weakness in Chibaku Tensei. As Madara's jutsus are the same as Itachi, he must know Madara's weaknesses as they are HIS weaknesses.
So it comes down to who can use their knowledge better, and seeing as Itachi is clearly shown to be much more intelligent than Madara, Itachi outwits Madara. If this fight was about strength, Madara destroys Itachi, probably every ninja there's ever been, apart from Hashirama and SO6P.
On top of all this, Itachi's Susanoo is virtually unbeatable. Also, Edo Itachi has a lot bigger chakra reserves than the Itachi that was alive, as alive Itachi was nearly dead anyway when he was fighting "Saucegay", prolonging his life with medicine just to be alive to face "Saucegay".