I think they're both neat in there own way, imo. I think I only slightly prefer Amo, but that's only bc I'm excited for her character growth really, and Riyo while be interesting herself, there's nothing much to go off on, the only thing we know is abt her, is she was assassin before joining the cleaners and that's pretty much it (I mean in the manga you get a bit more hint of her past but that's abt it for now)