Is the anime Knights of Sidonia really just Attack on Titan in space?
Yes. Yes it is. And you should watch it.
I first came across Knights of Sidonia because Netflix kept advertising it on their splash page, and so finally I decided to give it a try. It was a bit weird at first. The animation is good, but it sort of combines several styles, including CGI and cel-shading, and it reminded me of a video game more than anything else.
The world had very interesting rules. The idea of photosynthesizing humans was cool. So was the third gender, but they haven't really explored that very much yet.
Any anime with giant monsters that kill humans and an absurdly high death rate for named characters was bound to be compared to Attack on Titan, and while that's probably unfair, I think it's inevitable given Attack on Titan's popularity. Besides, many of the themes - militarism, pragmatism, extinction, loss of humanity - resonate across both shows.
Watch and see for yourself!