I think both they have made great contribution to Africa and also some mistakes. Gadaffi did achieve a lot for example allowing plenty of Africans to live and work in Libya. Over 2,000,000 Africans were working there during his time as President, but he did wrong in other instances for examples when he started to mix religion with politics and also supporting people like Charles Taylor in Liberia, Idi Amin in Uganda Against Uganda people wishes, destabilize Sierra Leone and trying to topple Chadian, Malian Sierra Leone and Sudanese Government which have created instability which still persist today.
He supported Janjaweed group who were killing dark skin people just because they were dark in Darfur while at the same time he was supporting South Sudan because he didn't like Sudanese Government. It would be great if he would stick with socialist and Pan African ideas from start to finish.
Mandela did great things for South Africa and Africa. You got to understand South African were super power in economics and military terms in Africa, it was absolutely necessary to get rid of them in order to rediscover African Manhood, prestige and confidence. Because they could destabilize any country in Africa.
And Mandela did that during his youth, starting youth wings of ANC, he went on campaign of bombing Apartheid infrastructure in South Africa until he was captured and convicted. He realise early on that talk alone will not work. Then he change while in prison. I guess you can say in his youth he was like Malcolm X and in later life he was like Martin Luther kings Jr. Both of his personalities have their place in the struggle.
I just think he should have done more when he came out of prison like build more houses for the poor, provide education, creating more jobs for people who have suffered for so long. Transfer some of the wealth from minority to majority. He fails in that regards to fulfill promise of freedom to the poor and disposed.
In the final analysis they both did some very good things and fall short in other things.