I think that part of the reason why Biko's success and Mandela's success differs is because their predicaments were different. For example, Mandela's success comes, in large part, as a sustaining symbol for his people while being imprisoned. Being able to endure and live through this tortuous period is what makes him so invaluable to those who follow him. The fact that he outlives Apartheid is an enduring symbol of hope to his followers, and something that almost proves on a divine level how he was on the side of right and defeated the sources of might. For Biko, his success lies in the fact that in appreciation of the struggle, there are many who sacrificed. Mandela's success lies in this overall vision, while Biko's is more a part of a cumulative effect.
On a more theoretical level, I think that both leaders approached the idea of a world without apartheid on different levels. Mandela and the ANC were more activist from a military standpoint, being able to employ the term "freedom fighter" in a very direct and literal manner. Mandela and the ANC saw the struggle as needing to take on political and military forms, and success was defined in these terms. Biko was operating on a more cerebral level, seeking to change the identity of those who suffered under apartheid in a more intellectual manner. Biko's employment of the idea of "black consciousness" and his appeal to the univeristy student would be representative of this. Biko was much more of a socially mobilizing force, and this is where his greatest success lies. His work and inspiration of Soweto schoolchildren is both a representative of his power, and a simultaneous representation of the White government's brutality. In Biko's success, one notices a definite social mobilization and not as much of a focus on political or miltary rule, a point of differentiation and vision of success between he and Mandela.