As everyone has already said, Lincoln, though he himself wasn't exactly a pro-black rights person, was surely a much better person for his time. You can argue Lincoln words when he claimed he was pro-white supremacy, but his actions were all pointing to the opposite. In case people forget, the South seceded because they were afraid Lincoln was going to abolish Slavery thanks to his House Divided speech. Lincoln couldn't exactly say "I love blacks, blacks should be free" in 1861 when both the North and South were pro-segregation and the south was still pro-slavery. He had an election to win, but his friendship with blacks and his value of their personhood was present in his actions. He just was a slimy politician...
Gandhi, on the other hand, was a racist and a tool. Forget the fact that he called black South Africans "kaffirs", Gandhi called them "like animals in a zoo" and was vehemently opposed to people comparing the plight of the Indian people to that of sub-Saharan Africans. He said they were different, Indians were civilized and Blacks were not. Keep in mind, Gandhi was a pompous rich t*@t who only wanted change when he was denied a seat in the better side of a train. He wasn't altruistic or magnanimous, he was self-centered and egomaniacal.
Now compare them to MLK, a man who fought for equality for all--blacks, Asians, jews, women, the poor. MLK's lone vice is he suffered from infidelity in his marriage, but most great men often do.
Lincoln for this reason. They both accomplished great things but they are also measured on higher pedestal than they probably should have been granted. Like any other beloved historical figures when tend to only see the good things regarding their character, and turn a blind eye to their more unflattering qualities.
Really depends on what is being written. If it is a mystery novel for example, first person is the way to go, but if it's something like an a fantasy tale with a bunch of characters than third person would be my preference.