Both can win depending on the situation. Obito would definitely have the edge with Kamui and Izanagi but Itachi has Tsukuyomi which should still affect Obito even if he hasn't materialized. Not to mention he has Izanami which counters Izanagi. Also if Itachi uses Tsukuyomi, Obito is finished. However if Obito goes straight for the kill by using kamui, I think he should be able to succeed. Given time, I think Itachi should be able to find a weakness in Kamui. If he gives Itachi too much time, itachi wins IMO. Even Obito said in the manga that if Itachi knew more of his secrets he would be dead by know.