One of the most notable differences is that Biko is killed while in custody. Steve Biko was one of many Black leaders in South African history who was killed by the South African government under the repressive practices of Apartheid. Nelson Mandela, also imprisoned and tortured, was not killed. I am fairly certain that the South African authorities would have had little problem had Mandela died while in custody, but a difference between both leaders is that one was killed and one was not.
Another difference between both leaders was their focal points. Biko was more focused on student activism and ensuring that students become the leadership point for change in South Africa. Mandela believed that students had a role, but his role in the African National Congress was more comprehensive, seeking to speak for more Black Africans in South Africa. Biko's focus on students was something he saw as being able to filter to more of society, but his emphasis on students can be seen as a difference than the approach that Mandela took in his efforts.