What is the difference?
Unabridged books are the entire book. Abridged books are a shortened version, with some elements removed.
When things are abridged, whoever does it makes sure to not remove anything important to the plot, but they do remove a lot of things like lengthy descriptions, or conversations between characters that don’t contribute to the plot.
Basically, abridged works, whether audio books or regular books, are just shortened considerably. That makes them accessible to more readers, who either don’t have the time or the inclination to read or listen to the original.
I always go with unabridged if I have a choice. Books are a type of art, and to get the full experience you need to take in everything the author wrote. It was included in the book for a reason, however small. Taking anything out makes it into a different book, in my opinion.