The variable nobody warns you about: the narrator can make or break a book, independently of the book.
I have abandoned books I wanted to read because of narration, and finished books I would never have read on the page because the reading was so good. It is closer to the difference between two productions of a play than most people expect.
What to check before committing:
Listen to the sample. Every store has one. Two minutes, and it catches most mismatches.
Author-narrated is a coin toss. Excellent for memoir, where the voice is the point. Frequently poor for anything else, because writing and performing are different skills.
Full cast productions are wonderful for dialogue-heavy books and distracting for interior ones.
Accents and languages. A narrator doing accents badly across a large cast is worse than one doing none.
One practical thing that resolves your first worry entirely: do both. Own the text as well for anything you care about, listen on the commute, and glance at the page for the parts you want to keep. It is the arrangement I ended up with for the books that matter, and it removes the choice you have been framing as a compromise.