Renton and Shingo are very similar in terms of drawing and personality. Both have messy brown hair and green eyes; both influence the other members of the organization. The mechas are both shrouded in mystery. Both leads fall in love with a mysterious girl.
Because they have simular sitiutation of "boy meets girl". Renton and Shingo have almost exactly same expression when they first met Eureka and Mana. Eureka and Mana chose Renton and Shingo to be their "partner". But the thing is different is that, Eureka seven have kind of less fighting and more love-y and Eureka and renton do even kiss in the end.
The "Unseen World" of Shinreigari:Ghost Hound is similar to resonant sympathy of Gigantic Formula.
It is true that isn't the same but in both is the notion of leave the body to be able to watch the reality. The Unseen World is a notion more complex while resonant sympathy is more like the capacity of connect some consciences and watch the past.
Both are mecha series that have mechas that aren't simply engineering products but are like gods who were awakened.
Interestingly, both mechas are called Susanoo.
In addition, the two series are featuring by children.
Both series have children fighting in battles with mechas. In Kishin Taisen Gigantic Formula are big mechas descendants of ancient giant idols while in Danball Senki are more small and not require be inhabited.
Also in two series there is a moment in where the children find out the truth about the war that adult people hide.
Both Anime are Really Interesting and Both of them are Talking about the Greek Methology, Where in Gigantic Formula are talking about the Greek Gods, While in Sousei no Aquarion are talking about the Stars Signs,Zues and Angels. Well u will get the point when u watch it.
Both of them are similar where they Combine to become more stronger, but in Gigantic Formula they kinda combine and use their enemy weapon or parts when they defeated there enemy.
Ohh i kinda forgot to mention that, Sousei no Aquarion has more Comedy than Gigantic Formula. But Its Still Worth to Watch.