well the difference in the two are:
the sunni's claim to follow the right 'path' which is the sunna. the sunna is everything narated by the prophet as long as the transmitters are trusworthy.
the shi'ite believe that only decendants of the prophet can be invested as leader and they keep an official list of those who should hgave governed the muslim world. most shi'i belong to a group called the twelvers. which are twleve imamsfollowing on from ali (the fourth caliph) to a young boy who dissapered in iraq. they awaits his return as the mahdi and untill then they are guided by its mujtahids(ppl qualified to make a legal ruling) like the ayatola (high ranking shi'i leader).
If you're discussing these variants in ethics then you could also mention that Sunni Muslims use the Qur'an and Hadith as sacred texts, whereas Shi'ite Muslims use the Qur'an, Hadith and Adab.
For decision-making, Sunnis use Qiyas (legal precedent) and Ijma (consensus of Muslim scholars), whereas Shi'ites use only the Ijma.
i'm doing the different variants for rites of passage an drites of public worship (ie. janazah and ramadan respectively) our teacher said there is no way we culd get full marks without writing about the differences between variants ... and i can't find my notes!