Izuna didn't make sense to me because Izuna was portrayed to be a soft hearted and honorable guy that died to give his clan glory(giving his eyes to his brother and dying to protect his clan). Izuna was dead buried so I thought it would be impossible for him to come back because Madara would had to have died in order to do so. Obito was more likely over him since we knew nothing about how Rin died and Obito was pretty much never buried and left under that rubble presumed to be dead. Also it was made VERY obvious that Tobi was Obito and people didn't want to believe it because the evidence was too straight forward. I was personally hoping that Tobi was the elder son of the So6p. Now that would have been very cool.