Quality is not, by it's nature quantitative, so it will always be both subjective and a moving target. So what does "quality" mean in the context of content marketing?
As much as I would like to say "Quality is more important" it honestly doesn't matter how good your storytelling is if you're talking to yourself. Compelling, unique content brings in an engaged audience, and nothing draws a crowd like a crowd. To achieve your goals you'll need quantity in audience and quality in content.
When you and I read a newspaper article, novel or blog post, we bring our own bias, opinions and experience to bear on what is ostensibly someone else's bias, opinions and experience. One should not presume there is a valid way to measure that - if one does, one had clearly lost site of their own bias, opinion and experience.
For instance, I loved Lord of the Rings the book and thought to movies and incoherent mess. My relatives loved the movies and though the book boring. Which narrative has the best "quality"? The books which have sold millions of copies over decades, or the movies which made millions of dollars? Both are high quality when judged by certain criteria, and perhaps not so high when judged by others.
Quality is judged over time: I've been writing a blog for over a decade now and I'm still gaining new readers. But some people vociferously disagree with my opinion, so they say my blog is poor quality. In this case, "quality" is the word we use to describe things that agree with us. ^_^
When you're selling the story of your business, "quality" means "good enough to bring someone back."
Quality content will sell consumers on your brand, your products, your commitment and on you.
In content marketing, "quality" is the impression you leave on potential consumers that brings them back as customers.