Besides immortality, these two seem to be different, with one more bestial and the other more polished. This can be explained easily enough, however, as King Kull was kept in suspended animation for centuries while Vandal Savage lived through said ages. This meant that Savage not only observed but adapted to the ever modernizing environment of civilization, a foreign concept to Kull.
Although Savage first operated in the late 1940's, tormenting first Earth-Two's Green Lantern before joining the first incarnation of the Injustice Society of the World, Kull debuted into the modern age in the early 1950's to repeatedly plague Captain Marvel of Earth-S. Each madman was a genius despite coming from a prehistoric race of men different from standard humans, now the sole survivors of these long-lost peoples.
Decades later in the 1970's, Kull would recruit the villains of Earths One, Two and S in a campaign of terror that led to the modern formation of the Squadron of Justice on his world. Similarly, years before this, Savage assembled the Justice Society of America who then escaped the prisons he made for them, and were henceforth reactivated as a team. On this occasion, Vandal met Earth-One's Flash, as had Kull during his cross-dimensional campaign. Shortly thereafter, Kull would become a member of the Monster Society of Evil, while Savage decided to operate apart from his fellow criminals.