Haechi is also about one of the greatest King in history like The Great King Sejong. However, the later is a very detailed and sometimes boring recount of Sejongs childhood to when he became King as well as his accomplishments. I liked it because it was a truthful recount. Haechi is about Yeongjo childhood to when he became King, it doesn't delve into much detail on what he accomplished and thus this drama was more dramatic and entertaining.
Both are happening in the same time frame. In both Kim Sang Kyung and Kim Yeong Cheol are playing the same characters, Kim Sang Kyung plays king Sejong and Kim Yeong Cheol - king Taejong. In The Great King Sejong Jang Yeong Sil is an important supporting character, while in Jang Yeong Sil he is the main character.
both is about the main character's journey to become the King. Since both drama are produced by KBS, the way is tells it's story it's similar. It's also nice to see Kim Yeong Cheol and Kim Gab Soo, playing king and subordinate in both shows before our main character succeed his character as King. The Great King, Sejong mentioned about Goryeo a lot, to learn about the history you can watch Emperor Wang Gun to understand part of the story.
