Brotherhood: Final Fantasy XV

Both prequels to popular Final Fantasy franchise titles. Both give the heroes and playable characters additional backstory. Fans of either game are likely to love seeing their favourite characters from the game in an anime adaptation. Both have beautiful art styles. 

Both provide additional animated content for the worlds of their respective Final Fantasy games. Advent Children acts as a sequel to the iconic FFVII whereas Brotherhood is a prequel to the most recent FFXV. 

Both share the same art style and provide further insight//lore to the world of FF XV. Whereas Brotherhood gives us some back story on the 4 protagonists of the game, Episode Ardyn gives us some well needed back story to the main antagonist of the game. Fans of Final Fantasy XV are sure to enjoy both. 

Both animes are prequel animes based on a video game franchise  

Both the MCs kinda give the same vibe. Both come of respectable high class families and are kinda spoilt by the facilities they have been getting since childhood and both hate responsibilities and are kinda lazy to abide by the rules. 

Both promotional ONA's set in the same canon/universe, the world of Final Fantasy XV. Both promote the game and intend to drive up hype/attention towards the series, the Fan Kansha Tokubetsu Movie is also promoting XV's prequel movie Kingsglaive. 

Both being based from popular jrpg franchises by Square Enix and also, both share a focus on the bonds of friendship as well as a stylishly cool cast. 

Both not only have "Brotherhood" in the title but explore it deeply as theme covering the kinship, strife and love in brotherly bonds. Both series villains are demon-esque in the characteristics and appearance. 

These both expand on the story of mainline Final Fantasy games. Brotherhood serves as a prequel to the more recent FF XV whereas Final Fantasy (OVA) continues on showing the aftermath of the game FF V. 

Both these animations provide added lore to a mainline Final Fantasy game. Advent Children Complete is the definitive way to watch some of the post game events from 7 whereas Brotherhood serves as a prequel to 15 and gives added backstory to the main characters. Fans of the FF franchise are bound to enjoy both. 

The series are similar in that demons are taking over the kingdom/empire and people use magic/weapons to stop them, but what's really similar are the main characters--throughout the whole thing Leon just reminded me of Noctis, not just angst-wise but especially with the whole chosen-one thing and those rings they have that give them powers. Other than that they both have a similar vibe. If you've played the game or seen the ona you'd probably think so too. 

I believe these are both by the same studio, share similar animation styles & both provide added lore to popular mainline FF games. Denzel offers a post-game side story of sorts to 7 whereas Brotherhood is a prequel to 15. Fans of the franchise will enjoy their time with both.