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.