3.835 out of 5 from 83 votes
In both there is some sort of bodyguard\detective high school and in both the main lead is far more badass the he lets other believe (well, in Hidan no Aria it's more of a matter of a switch being flipped). Not to mention that a character with multiple personalities appears in both. If you liked one the other should be right up your allay with all the gunfights and stuff, though Hidan no Aria has less action than Gun x Clover.
The similarity between the two series comes mostly from the fact that both feature a badass male lead who pretends to be weak. Also both take place in some sort of special high school. Content wise Gun x Clover has some strong ecchi which Mahouka Koukou no Rettousei does not. The story in Gun x Clover story is not it's strong point either. Nonetheless if you are after the strong guy pretending to be weak theme, then both pf those series should prove to be worth reading.
Köinzell is a master swordsman who, along with thirteen other heroes, was sent by the Szaalenden Empire to block an invasion of their enemy Wischtech. Along the way three died, four betrayed the others and seven defeated the traitors and returned as heroes. At least, that's what people believe. In reality the ‘Seven Heroes' were the real traitors, defeating the four true champions after the seven had fled the battle in cowardice. Köinzell has bided his time for twenty years, honing his skills and recovering from the brink of death so that he may avenge his fallen comrades. The time for vengeance has finally come and all who stand in his way shall be dismembered without hesitation, but it won't be easy as everyone believes the lies of the traitors. With all the soldiers in the land and the public against them, Köinzell and his ragtag allies may have to sacrifice everything if they are to prevail against the might of the entire nation.
Both series sport a badass male lead who happens to look like a child and thus being underestimated a lot. If you liked the guy in one series, the other should prove interesting. Keep in mind that both series have some mature content (Ubel Blatt probably has more of it than Gun x Clover, but at lest the story is more coherent).