To be clear, once the human-avatars are 'born' and become their cities, they become (I think?) functionally immortal -- there's reference to the avatar of Hong Kong living through the Opium Wars, for example -- and things that impact their city impact them, but also vice versa (São Paulo gets cosmically punched and it causes a small earthquake.) So there is a fair amount of complexity/ambiguity there in how much the avatars decision-make for/act upon the city, and how much is the city acting through them, if that makes sense?

But still, in a lot of ways I like the concept of 'this avatar is a human-looking personification rather than The Person Who Got Chosen' better too, because it allows for that avatar to be sort of all things to all people within its borders at once -- it feels more fluid and like it has more room to encompass wide variety of perspectives it contains. On the other hand, in practice Hetalia (from what I know of it) paints nations in extremely broad strokes and The City We Became is by its nature a lot less cartoony and more interested in digging down into the complexity and inherent contradictions of any attempt at defining local 'character', if that makes sense.