I preferred reading books when my focus is for deepest understanding of content. On the other hand, I preferred listening when I want to develop my pronunciation and listening skills. And I believe the two types of books (audio and printed books) have own purpose to our development. Moreover, as we can observe, all of us have own differences in learning some want to see printed symbol or they like to read however some people want to listen audio for their study.
According to education, there are three types of learners the auditory, visual, and kinesthetic learners. And I guess audio books are made for auditory learners and printed books are made for visual learners; with these, learners can easily learn and develop their skills by choosing their preferred learning materials appropriate to their learning style.