There is no Sunni and Shia in Quran. You read the Quran and you will not find a single word of Sunni and Shia, Allah says in Quran “Hold to the rope of Allah strongly and be not divided” You have to follow the Quran and authentic Hadith.
Shia came later on because of political differences, it has nothing to do with Islam. In Islam, there is no sect, Allah says in Quran “If anyone makes sect in the religion of Islam, Oh Prophet you have nothing to do with him, Allah will look after his affairs”
There are many verses in the Quran which says makings sects in the religion of Islam is prohibited, there is no sect in Islam and all these are because of political difference that came but in Islam, there is nothing like Sunni, Shia, there is only Muslim, Muslim is a person who submits his will to Almighty God.
Now one can ask that which belief is more correct and the simple answer to this question is, the belief which believes in Quran and authentic Hadith is correct.
The person who believes in the teachings of Quran and authentic Hadith is correct, If anyone gives you the reference from Quran or authentic Hadith believe it but if anyone tells you that my shaikh said this and my shaikh said that, first you have to see whether it is authentic or not, if it matches with the Quran and Hadith, I have no problem but if it doesn’t match than I am sorry.
Allah says in Quran “Obey Allah and obey the Messenger” the verse doesn’t stop here, it continues “But if the people of knowledge defer, go back to Allah and his Messenger” If two scholars say two different things, check up which scholar matches with the Quran. The one who follows the Quran and authentic Hadith is on the straight path and is a true Muslim and other is not.
May Allah protect us all.