I think mainly the different dynamic possibilities between a Mob who never got the guidance and reassurance he needed during his formative years (and thus lacks a grip on his emotions + didn't develop the sense of morality of his canon self) and a Reigen that lacks the life experience of his canon self, which can show him to either still be excited about his interests with naivety or exploring his potential back story and family dynamics.

I also really just like the concept of "bratty, overly loud and needy of approval teenager gives lectures to emotionally stunted grown adult who is fully capable of killing and is fiercely protective of his disciple", haha