At this point, one vote for Darwin… And not by much! The two are really very much on a par!
Darwin made personal observations, went out into the field, actually dug up the bones, actually watched the tortoises and iguanas. Then he got theoretical, and then went back to being practical, by raising pigeons.
For me, what tilts the balance, is the mathematics. Einstein, obviously, was highly mathematical, and damn brilliant at it. But Darwin worked out the statistics of heredity in the abstract. He even worked out how heredity had to be discrete: he was well on the way to discovering chromosomes – indirectly. This, in my mind, is staggering.
Einstein based much of his work on what others had done or were doing, but Darwin worked almost entirely alone. This doesn’t really undermine Einstein’s genius, but it suggests that he was supported by a diffuse team of researchers, an advantage Darwin didn’t have.
And, to be sure, both men’s discoveries were “in the air.” Wallace actually scooped Darwin, and, if Einstein had died young, someone else would have put all the pieces together. Evolution and Relativity were waiting to be discovered. Evolution actually was discovered before Darwin, and Lorenz and others had prepared the building blocks that Einstein stacked together to discover Relativity.
Just a personal thing: I’m more impressed by Darwin than by Einstein.