This is a tough one, for a variety of reasons.
Darwin obviously impacted the world more than Lincoln. Duh.
But Darwin did so because he was the first to get to an idea that was inevitable, i.e. the scientific discovery of evolution. He wasn’t the only one to get there, he got there first, and he got there well.
Had Darwin never been born, someone else discovers evolution, because ANYBODY could have discovered evolution.
That said, evolution as an idea is so consequential, so radical, that it changed the course of human history. Without evolution, God doesn’t take the hit. No hit to God, and Christianity does not decline in the West, nor does Nazism or Communism rise in the East. For no Darwin = no Social Darwinism, after all (i.e., the corruption of science to justify racist/utopian fantasies).
Darwin came along at the right time/wrong time for all of this-- throw in Nietzsche and Marx, and you’ve got the 20th Century (all its conflicts, and many of its successes) largely explained right there.
That said. . .
No Lincoln means the Civil War probably results in the breakup of the U.S. Maybe not, but given his genius, let’s assume that happens as the counterfactual.
What does the world look like today with the United States? What does the world look like today with representative democracy?
The U.S. dies in the 1860s, the idea of representative democracy almost certainly dies. Maybe it would have survived in Britan for a while longer, but would Britain have survived? Not only do you have the Prussian/Imperial Germany counterexample for other nations to emulate, but once communism and ultimately fascism shows up on the scene, what is there to resist it? Not just talking about the fact of the U.S. power to resist/help others resist those tyrannies, but without Lincoln, the idea of America no longer exists. The “example to the world” is gone for good.
And who wins then? The nations that took what Darwin wrote, and corrupted it to their evil ends, for the extermination and subjugation of races and classes they determined were scientifically inferior.
So, perhaps we should look at it this way-- without Darwin, evolution is still discovered. Without Lincoln, however, Darwin’s discovery is used to permanently darken the world, with nothing to stand in its way.
I know that it’s popular in many circles to trash America, put the old lady in a smaller place, etc., but for all the flaws and faults and mistakes and misdeeds, a 21st century world without America-- without the idea of America-- is IMHO a far more horrifying scenario to contemplate than a world that hasn’t gotten around to discovering the origin of species.
So, my answer? I’m damn glad that we had both brilliant men.