I think the answer wholly depends on why (and what) you're reading. I mean the act of processing ocular processing activates a different part of your brain than one that processes aural processing. Like mthor said, it might be easier to be distracted if you are listening and are distracted by something in your line of sight for various reasons. But assuming you give it your full attentions and the point is to use books to educate yourself or to think critically, hearing it should be no different than reading it as far as intellectual stimulation. If you're going to read Mary Trump's book, you will be no more intelligent for reading it as opposed to listening to Meryl Streep read it to you.