Both manga belong to the psychological/sports/seinen genre. As such, they focus mainly on the psychology of their characters instead of the actual sport. Both have amazingly written, fleshed out and developed characters. They both have a unique artstyle, although Kokou no Hito looks gorgeous while Ping Pong looks eccentric. Even during the execution of the respective sport (Ping Pong and Mountain Climbing) the characters are given the focus, with lots of imagery and symbolism interrupting the actual sport
Read both, you won't regret it!
Both are sports manga by Taiyou Matsumoto, and as such, they have many similarities. First off, both manga have a similar art style, which instead of words, relies mostly on imagery and metaphors. They also have an unorthodox approach and are unique in the way they show sport matches, opting to use sport as a means for the main characters to gain a better understanding of themselves and their lives.
Both are sports manga, which do not focus on the sport, but on the character development. Both feature energetic characters, who gain new experiences and meet new people while practicing their sport, leading to a better understanding of themselves, their talents, and their surroundings.
Both are manga by Taiyou Matsumoto, therefore, they have similar art styles and character designs, as well as story elements. Both anime focus on a duo of main character with contrasting personalities. The plots also contain a lot of metaphors and character development.
Both Ping Pong and Hajime no Ippo are manga based on sports, with Ping Pong being about table tennis and Hajime no Ippo being about boxing. What makes these two anime similar is the character development: both Ping Pong and Hajime no Ippo focus on several character, all with unique personalities and skills. Both focus on exploring the motivations these characters have for practicing these sports. Ping Pong and Hajime no Ippo also deal with similar themes, like for example: talent vs effort etc.