The only thing Itachi has going in his favour is MS Genjutsu Tskuyomi and that according to Itachi himself can be easily overcome by someone having the same blood as him, which would be Fugaku and Sasuke. Other than that, Sasuke beats Itachi in terms of Susano'o techniques, he can form Magatamas, arrows, and sword from Enton flame, and create arrows from spiritual essence in Susano'o third hand.
In fact, he is show to carry Enton flame and Enton Arrow simultaneously and bring them to use as well. Itachi won't be able to hold CS for long, as it's quite taxing for the body. With this form, he wields the Yata-Mirror. All Sasuke has to is wait it out. His CS will drop down to a lower form, as any form higher than bones puts even additional burden on the body and saps the chakra even quicker, as we saw here:
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- Sasuke's ribcage version Susano'o completely withstanding Mei's Lava attack and Chogiru's Hiram Ekarai Release Attack:
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. Even after both of these attacks, his Susano'o appears unscathed -
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.
But Sasuke mentions it here -http://www.mangapanda.com/93-34416-5/naruto/chapter-466.html - that the prolonged usage of Susano'o is 'taxing on the body if used for a longer period' when this is just the ribs form.
Sasuke ribs form still protects him, but melts as he is already feeling MS leeching on his Chakra. But when he tries to jump to a higher tier, the Susano'o defenses crumble and the mist is able to breach here -
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and his chakra reserves grow weaker due to the jump -http://www.mangapanda.com/93-34416-8/naruto/chapter-466.html. Both Mei and Karin attest to that fact.
This shows that if the form is higher, it would sap chakra more and Itachi won't be able to keep up CS for long. Once it goes down, Sasuke can attack the Incomplete form with Magatama and arrows and it will crumble due to its less durable form and Enton's potency. Any Genjutsu thrown at Sasuke is met with equal resistance, hence Itachi's only advantage is lost.
Even if both resort to Taijutsu, then Sasuke and Itachi can both maintain a rib form in motion -
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and
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. Both have shown incredible speed feats and even if they don't decide to put up Susano'os then Sasuke is quick enough to counter Itachi's attacks with Speed as we saw we he easily dodged Bee and A's attack. Here Sasuke has the advantage as he can use Nagashi to paralyze Kakshi and can even use Taijutsu in conjunction with Kusaagi. Itachi will get bested in Taijustu due to his scanty list of out-of-susano'o techniques.
Sasuke takes this mid-difference max.
