3.916 out of 5 from 55 votes
Over a century ago, mankind was devoured by giant beings of unknown intelligence and origin known as Titans – creatures that eat humans alive indiscriminately and for no apparent reason. The remaining population has managed to survive the last hundred years only by building a multi-walled city capable of keeping the Titans at bay, training military recruits to patrol the perimeter and gather intelligence about their mysterious foe. Eren and Mikasa have lived a relatively peaceful life behind the city's walls, but when a massive Titan appears, smashing the outer barrier and unleashing a wave of terror, their lives are brutally changed forever...
Both series focus on a dwindling group of "last surviving" humans, who are fighting an overwhelmingly powerful enemy. The enemy in each case is very alien, yet has some mysterious connection to humanity.
Sidonia is placed in a futuristic setting while Shingeki is placed in a more medieval setting, yet everything else about the two series is strikingly similar.
Attack on Titan and Knights of Sidonia are so much alike that it feels like they were adapted from the same source material. The general concepts and plotting and many plot revelations are very similar. However whereas Attack onTitan is a medieval fantasy, Knights of Sidonia is a sci-fi mecha story.
So if you like the basic idea of these shows but prefer to see it told with an emphasis on horror, gore, despair, panic and general chaos, featuring an improvised chaotic approach to defending what is left of humanity and a heavy emphasis on action then Attack on Titan is what you want.
If you want a strong emphasis on setting and a more logical, coordinated, team based approach to the defense of humanity, with more deliberate logical plot revelations and a more focused overall story telling approach then Knights of Sidonia is what you want.
Renton Thurston is a young boy who helps out around his grandfather's mechanic shop; but despite leading a relatively boring life, Renton dreams of traveling the world. However, Renton's life as an average teen comes to a screeching halt when a humanoid mecha called an "LFO" falls down from the sky, only to land right in front of his eyes. From inside the damaged giant, a mysterious girl named Eureka emerges seeking repairs. Renton soon learns that Eureka is a part of the infamous Gekko State, a group of travelers who rebel against the government. He joins them on their travels, but what strange truths about Eureka await him?
Both series tell the tale of humanity's conflict with a strange alien force. In both cases, this alien force attacked the earth at some before the story begins, and sparked off an ongoing strife between humans and the alien attackers. Both series have many things in common, including the use of mecha, the fact that the aliens are extremely alien in both form and mentality, and the fact that the aliens try to take on human forms.