Cantarella, Cesare: Hakai no Souzousha Recommendations
They are both about the same person. Cantarella is more of a fantasy, while Cesare is purely historical.
Both manga depict the story of Cesare Borgia. However, that's about where the similarities end. "Cantarella" is more supernatural shoujo-fantasy, with Cesare depicted as a tragic figure, who wields demonic power at the cost of his humanity. Historical events are followed loosely, and Renaissance Italy is a sort of backdrop for the often romantic-themed conflicts between the characters, all told with beautiful, flowing artwork. "Cesare" is also a bit dark, but in a way that there's more focus on political atmospheres and struggles, and Cesare himself is more of a charismatic figure whose power begins with his intellect.
Both are about the historical Italian, Cesare Borgia, and the people, politics, and intrigues surrounding him. Cesare is a realistic story, whereas Cantarella is a more fanciful version, with shounen-ai, and some supernatural elements.