This is going to sound funny coming from an orchestral conductor, but I prefer audiobooks when I backpack. Perhaps it's that I work with music all day, and I never have time to read, so for me an audiobook IS a change of pace for me.
But it's more than that. I think backpacking, and the drive to the destination, are the perfect settings for audiobooks; they usually are several hours long, so I can really get into the text. With music, every 3-5 minutes when the song changes I am reminded that I am backpacking. But a good audiobook can really keep my mind off the trail when I get really tired. It also really helps on in-and-out trips, where the route up is interesting, but the route back can get a little long.