Really?
In the "primary-at-the-time" comics version, who has Batman ever actually killed? I'm genuinely curious, as I can't recall anything. I guess the closest I ever saw was when he fought Qayin (whom Bruce mistakenly believed was responsible for the miscarriage of his and Talia's baby) in the memorable graphic novel "Son of the Demon" wayy back in 1987, and even then I'm not sure Batman dealt the killing blow himself.
Superman, on the other hand, HAS killed in the "primary-at-the-time" comics version. He reluctantly but forcefully acted as judge, jury and executioner to the pocket universe's Phantom Zone criminals. The three villians had just went full-on Kryptonian and wiped out the entire population of Earth in the pocket universe and threatened to find and do the same to Superman's Earth. Superman first exposed them to Gold K to rob them of their powers and then exposed them to Green K to kill all three of them. Killing them affected Superman so greatly he began subconsciously masquerading as Gangbuster and then left Earth for deep space for several months because he was afraid of what he was now capable of.
Both stories have been retconned, but the Superman one was definitely in-continuity for a long time.
Superman is (ridiculously) more likely to kill.