Terra Formars: Bugs 2-hen

Both anime have a dark and serious theme where humanity is forced to face a much larger and stronger enemy than themselves. The enemies (Titans in SnK and Mutant Cockroaches in Terra Formars) both mercilessly slaughter humanity like if there is no tomorrow. Humanity is not completely screwed as our main cast in both anime do have a trump card at hand. 

Both are action series where humans fight overpowered supernatural enemies and tryes to survive. Some humans also have power similar to the enemies. While Shingeki no Kyojin is fantasy, Terra Formars OVA is sci-fi. 

Both have much gore and blood. Even the best ones does not survive. In both the enemy is bigger and stronger, and have unknown way of communicating. In both they use genetic engineering to gain andvantage over the enemy. In both the enemy is unsolved enigma. And they cannot win unless they solve it. Of course they have plenty of diferences, but the gore is what it makes them similar to each other.