Leaders are generally determined by age (Hashirama, older then Tobi, Hiyashi older then Hizashi) and heritage. Madara and Izuna were the children of the previous leader(s) so they were the clan heirs, however Madara was older so he inherited the title of leadership in the Uchiha clan.
I voted the draw option. One: MS Madara was sort of like Sasuke, and I mean that in he spammed his MS to the point he went blind. Izuna we could assume, had more sense with his eyes since we didn't hear anything about him going blind until Madara became a selfish brat (like Sasuke) and took his brother's eyes.
Now it was always stated that the two were rivals and constently fought against each other but there was no such winner in their bouts. Now Madara with EMS could take down his blinded brother (given the fact that he would have Izuna's eyes), but if we go hypothetical, and say Izuna still had his MS, Madara would still win with EMS. However both of them with their Mangekyou seem just like they were, when they were younger, equals so I said draw.
