When Koh was eleven years old, he lived a quiet and peaceful life, delivering sporting goods for his family's store and batting frequently at the Tsukishima Batting Center. Though Koh had no interest in baseball, he started the play the sport anyways after a series of events, much to the delight of his best friend, the beautiful Wakaba Tsukishima. However, soon life dealt Koh a tragic turn, changing him forever. Now, years later, Koh attends Seishuu Academy and is soon pulled back into the world of baseball. Alongside Wakaba's talented sister, Aoba; old friend and fighter Nakanishi; and plenty of new teammates and companions, Koh will once more pick up the pitcher's mitt and see if he has what it takes to be a champion.
Those two shounen sports stories are very similar. In both tiles main character isn't interested in the sport at the beginning. It all changes when tragedy strikes, and a very dear person to him dies. From that moment he starts to change himself, in order to become an oustanding player.
Apart from the main characters those titles also have many similar situations. For example in both mangas there are 2 clubs for the same sport in the same school. At some point they clash against each other, to determine who should represent the school. Also both titles have realistic approach to the sport theme.
Both of these manga are done by the same person and it's extremely obvious. Several of the characters look identical, and even act similarity. They both also share a similar theme (a girl in baseball). However, although they both have the same atmosphere, Idol A doesn't go quite as in depth as Cross Game does with its characters and baseball games due to it's length. Fans of one would enjoy the other for sure.
Satoyama and his best friend Kawakami both like the same girl - Mizutani, a tomboy whose father owns a boxing gym. They both join the gym to get closer to her, but unfortunately she has a bad relationship with her father and for some reason won't talk about boxing. When she sees Satoyama's punch, however, her heart begins to change; she had given up on boxing, even though she was a boxing prodigy, because a woman could never compete with a fully-grown man. But perhaps she could train Satoyama to take on the world in her place! Satoyama doesn't really care about the championship, but if it means Mizutani will spend more time with him he'll do anything!
Replace the baseball in Cross Game with boxing, and you have Katsu! They're both done by Mitsuru Adachi and are extremely similar. They have the same humor and some character designs are shared. If you're a fan of one, I'd strongly recommend the other.