Similar story and setting, where highschoolers fight eachother to reach the top and to be the strongest fighter or "king" of the school.
Both are of a similar theme, and both are damn good. (Hard Romanticker is a little slower pased, but still a good watch)
Both series are based on successful mangás of the same genre and have the same high spirits of delinquent students fighting to reach the top, each one for different reasons, and that leads to creating meaningful new friendships along the hard way.