This may be sound ridiculous..considering how far different the theme concept of both anime.
So? Why you recommend each other?
The story and the feeling brought.
Both of them have a similar kind of fate and bit by bits of storyline.
They are connected with Gods, powerful being that hold control to the world and how the heaven and hell hold a powerful control that determines the whole story, simply to say...JUDGEMENTS from something beyond humans.
Both of the series sacrificing the most dearest character of the main character which in the leaves a scar to us (the audience) of how the story could end with a happier kind of ending yet it is not.
Altho in that regard Shingeki no Bahamut have a more tense and tick plot while Noragami Aragoto a little bit more casual and slower.
Both have a concealed and surprising which will led us to switching opinion with some protagonist turns into antagonist and the other antagonist turns out to be protagonist.
Both have a majestic way of showing how enormous and Godly a battle between Gods, Demons and Human and...as well as showing a STRONG emotions of sadness losing someone dear to you. That pain in your chest is real.
Well, enough to say....if you like those 'pain in the chest' feeling of losing someone dear, both share the exact thing you looking for with an extra of GRAND BATTLE between HEaven and Hell.