Jinzou Ningen Kikaider The Animation

For starters, both shows star robots and feature a large cast of them interacting with humans. In addition, both shows are almost entirely grim while featuring a tormented protagonist. If you can get past Kikaider's retro animation/style, and you like Casshern Sins, you'll most likely enjoy it. 

Both Kikaider: The Animation and Garo: The Animation are based on Tokusatsu TV series. Garo is more darker and violent compare to Kikaider. The main character in both anime henshin (transform) to fight other monsters and struggle against other evil forces that threaten to hurt the people that they love. 

Both anime feature a robot theme superhero fighting crime and protect innocent people from evil. They both feature a mystery surrounding the main character. If you liked the one, check the other too. 

Both involve a child and a detective. Both are old (Metropolis is newer, but it's based off of the creator's manga [the godfather of anime, Osamu Tezuka, created both this anime and the manga[1940s] it is based off of). Both leave a lasting impact. 

Both have little kids who play an important role to the story. They help make the MC a better character. They add spunk. 

Very similar animation, character design, and story telling. Concrete clearly pays homage to Kikaider in several ways. In an early episode the main character of Concrete (Jiro) is seen acting quite a bit like the main character of Kikaider (also Jiro). Both characters were unaware and/or rejected their true natures and had a lady helping them accept it. 

Both anime have a organization who is after the main character for a similar purpose because they have more strength than ordinary. There is also a similar romance relationship as they both are in love with a certain girl/woman. 

Dark Retelling's of Shotaro Ishinomori classics, each one retells the story of a robotics based henshin hero with a more darker tone than its original. Although the dark I'm referring to for The Skull Man is more of a horror story dark while the dark for Kikaider the Animation is more of a emotional and a tiny bit film noir style dark 

Tokusatsu Hero Anime, while Jinzou Ningen Kikaida isn't a SD Anime like Kamen Rider SD Here, its still a Tokusatsu Hero Anime of a legendary series in Japan.  

Tokusatsu Anime part of the Ishinomori Kikaider/Inazuman universe  

Tokusatsu Hero Anime 

Both were initially created by the late great Shotaro Ishinomori. Both are about Androids with somewhat dark tones, namely the loneliness and melancholy mood by being an Android/Cyborg.