The most obvious difference is that Mandela lives to assume Presidential leadership and political power and Biko did not.  This might also reflect a similarity in that both were victim to imprisonment and the "enhanced interrogation" that goes along with it.  In terms of their leadership positions, the absence of one led to the rise of the other.  When Mandela was imprisoned, it caused a void in the leadership of Black South Africans against Apartheid.  While Mandela was imprisoned for his militant approach that was rooted in the idea that military action can lead to political change, Biko was arguing for a mental and spiritual consciousness change in how Black South Africans saw themselves.  On many levels, Biko argued that the consciousness and self perception of Blacks have to change before political leadership change can be effective.  Whereas Mandela argued that political change is its own end, Biko argued that Apartheid will come to an end when Blacks begin to understand that their own sense of self is beyond being controlled by Apartheid tactics, it will come to its own end because Blacks "are not going to take it anymore."  This becomes the reason why the Soweto Uprisings, inspired by Biko, become so important in South African History.