So ...

Harry and Quirrell are partial backups of Voldemort, written onto the respective brains of a young boy and an adult man.

For Quirrell, being the Dark Lord is a core part of his identity; he identifies with that part of himself and its goals, namely personal immortality, which he believes can only be achieved at the expense of others. He believes this for good reasons, namely the underlying magical laws that give rise to the rules for potions and rituals: something of equal value must be sacrificed to gain a desired end: light for light, life for life. Magic is, in the end, zero-sum. Quirrell believes the zero-sum nature of the universe is cold reality, and that anyone who denies it is lying or deluding themselves.

For Harry, being the Dark Lord is not a core part of his identity, but a peripheral part ("mysterious dark side"). Harry identifies not only as a child of the Enlightenment, but as someone who wants to use his powers to elevate all of humanity. Harry does not believe that the magical laws are the way the universe works; he expects human accomplishments can be positive-sum. He believes this for good reasons, namely the positive-sum accomplishments of Muggle science and technology.

(Draco understands science in magical terms — as a sacrifice of belief. Harry would like to understand magic in scientific terms, but he hasn't had the time yet.)

Quirrell does not see how anyone could live forever without killing others to use their life, one way or another — whether by overwriting humans or draining unicorns. Harry figures it's possible through technological means, possibly augmented with magic.

It's also interesting that Harry is accustomed to thinking of himself as somewhat multiple — not only his "mysterious dark side" versus his normal personality, but also the aspects of himself that he thinks of as belonging to the different Houses. Quirrell, on the contrary, may well be accustomed to thinking of himself as a unified consciousness who projects a lot of façades.

This could just be a developmental thing; Harry's mind was more resilient at the time that he was imprinted with the Voldemort persona. Or it could be that Harry was too young to experience the imprint as a death of self and so abandon his previous personality.

Or, you know, there's Lily's sacrifice ...