Both mangas involve two princes of the underworld. Similarly, instead of being your typical, bloodthirsty demon, each have sympathy for humans and their fellow denizons, trying to build a world where both can live harmoniously.
Nurarihyon no Mago and Defense Devil both feature a young male protagonist who possesses an ability to transform, among other characteristics. Both mangas are lengthy, shounen, and are interesting due to their repective plot designs and character growth. Both stories boast a well designed world that is relevant to the main character and keeps you interested.
If it had to be said, even though both series have serious elements, defense devil felt more serious to me.