Comparing the two services for audiobook purchases on the iPhone / iOS devices I have both iTunes and audible audiobook purchases.
Pro: -download purchased audiobooks
-books are often cheaper(w/membership)
Cons: You let me know. Their app is even awesome.
Pro: -works with iTunes gift cards
Cons: -no downloading past purchases without paying full price again.
-no app with features like audible.
-no membership for discounts.
Overall I would say go with audible.