I am aware of a feeling that if Mandela suffered at the hands of the government that there would be a reaction akin to Soweto or Sharpeville. However, the Black Consciousness movement was arguably more prominent in South Africa than the ANC was (during its exile to the camps out of RSA) and yet Biko's passing is not attributed (in todays ANC narrative) to any major uprising (as far as I am aware). I assume this is because of the political/social climate of the time. I am hoping that someone can fill me on the more relevant details.
The context is probably the most important thing. Mandela was captured in 1962 and went to trial with the other people captured at Liliesleaf in 1963. At that point, the South African government felt it had decapitated MK; it was operating from a position of strength, and frankly had not yet arrogated the full array of police (including detention) powers it would have just fifteen years later. With the Rivonia prisoners "safely" imprisoned on Robben Island, there was no situation in which they would be exposed to the summary detention and questioning that led to deaths like Biko's.
Biko's killing while in custody was the product of a very different and much more fearful time for the apartheid government--post-Soweto. Biko had been questioned before, he had even been before judges. But then Soweto happened. The rising wasn't just June-July 1976; it rippled across the nation and threatened chaos every year on the anniversary if not at all the times in between. The number of deaths in detention grew rapidly after that date, and included not only Biko but other long-time opponents of the regime. Had Mandela been captured in 1976, he could very well have paid that price under interrogation. But he was already imprisoned, so he couldn't fall into the "grey area" between police detention and trial anymore where such extralegal killings happened. It's also worth pointing out that a variety of politically active figures were detained, tried (often in groups), and went to Robben Island or other prisons, so murder during initial detention was hardly a uniform policy or something the government really wanted--but neither did they take any great efforts to stop it, and in Biko's case the actual killers told the TRC that it was a very immediate thing. It's arguable that Biko's killing did stoke the fires of confrontation and violent action against the apartheid state just as you suggest Mandela's death would have--but we can't cleanly separate that from the broader upwelling of radical action that characterized the era after Soweto.
Welsh's Rise and Fall of Apartheid is what I always go to for general discussions of these points, at least in context of broader policy. The post-Soweto fear factor can't be discounted, because that was also the time when the armed struggle returned in force (MK began much more aggressive and even deadly action, for example) and SA's friendly neighbors no longer were there with the end of Portugal's empire and the impending victory of ZANLA and ZIPRA in Rhodesia.
[edit: note that there were other executions--notably of people accused of armed action, particularly if it had led to deaths--so the judicial system could produce prisoner death as well. But Mandela and his cohort had no deaths that could be pinned to them in any way.]