If I'm in a hurry, I use iTunes because Audible is so time consuming, all to save a few bucks. The enhanced audio sounds really good, but I tried to put it on an older ipod for a child and I had to drop down to a lesser quality to make it work. This article has screen shots showing the differences.
Comparing Audible.com and iTunes for Audiobooks
When it comes to audiobooks, Audible vs. iTunes is a microcosm of Amazon vs. Apple. Audible has more choices, and iTunes is easier to use for purchases.