Since I left Islam, I did notice that there were more shia apostates than sunni ones. The shia apostates were mostly from Iran. I have my own take on this which is that shia's traditionally are more exposed to speculative theology and concentrate less on "fiqh". The sunni's have also had a speculative theological tradition which is through their aqeedah. The difference though is that sunni's like to derive alot more from the Quran and Sunnah. From my understanding shia seem to be exposed to more rational thought in their theology, which gives them room to maneuver. The other reason particularly with the Iranians is because of the theocratic oppression they have seen over the past thirty years. Historically they have also not liked the Arab culture that was imposed on an already established civilization.
It is my hunch that Iran will be the first to flip against islam. They are a minority in islam, so the sword verses in the koran are particularly intimidating for them. As was mentioned in the thread they are a bit of the outcasts in the muslims world. They do not revere the original muslims and quite despise them: (i.e: khattab, aisha, uthman, bakr). And the Iranians hate the arabs. They seen islam in action for 30yrs.