From a personal bias, I like Olivia more, however, thinking about Machino's perspective, Sofia asks more questions about Machino and her childhood, her thoughts, her fears, her wants. She gives Machino choices like Machino giving Sofia choices when she first arrived. Meanwhile, everyone else's arc seems to center around themselves, including Olivia, who talks about their promise, her magic and her fear of the dark.
HOWEVER, I reeeeeally wanted Olivia ;_; I kinda wanted to subvert the "first love interest is the last love interest", it would've been great if Olivia, despite not being the first person Machino saw, was the one she really liked all along (sobbing), however, just from the way it was written and drawn, it was obviously going to be Sofia, and I'm not that mad about it, I'm just a little sad.