Both are just pure fighting series and I would even say Koukou Tekken-den Tough is even more amazing because it seems relatively realistic, at least more than Baki. So if you like all out martial arts series like Baki or Koukou Tekken-den Tough, check out the other one!
Both are sequels to masculine battle shonens featuring tournament style combat. These series also share a similar plot of traveling to a country filled with martial artists while in search of a cure to a dying character. Lastly, Baki and Shishimaru have a few father-son moments although Baki's relationship isn't as cheerful.
Both are sequels in which the authors' drawings improve greatly while also bringing more violent content to the fights. The matches of both MMA and wrestling are over-the-top but also share the same struggle of the main character living up to his father's reputation.