The top three sellers of audio books online are Audible, Downpour and Tantor. I have bought from all three and have had variety degrees of satisfaction.
Audible http://www.audible.com/ - Audible has a couple of big pluses and several huge negatives.
The pluses are selection and the ability to "whispersync" with Kindle books. Audible's catalog contains over 100,000 titles. Kind in mind, there are not all books. Some are radios programs with each episode counting as as additional title. When Amazon bought Audible, they began to match their Kindle ebook selection to match Audible's titles. Whispersync is a feature where you can read a book in a Kindle app and have it sync to the Audible version. Basically it allows you to stop and start reading and/or listening to a book across devices.
The two negatives with Audible, and they are big ones in my book, are price and format. Audible is expensive no matter how you select it. Their plans range from $14.95 for one book a month to $22.95 for two books a month. Their daily deals are usually under $6 but I haven't seen one yet that made me want to jump on it. Buying the books without a membership is just plain expensive. Amazon will offer reduced prices on certain Audible titles if you purchase the Kindle book. That is probably the best way to go. Otherwise it is just too expensive. The other negative is the format. Audible has a proprietary format that makes Apple look like opensource. You cannot convert it. You cannot share it. You can only play it on a limited amount of devices using Audible's own app or file manager.
Tantor Media http://www.tantor.com/home-consumer.asp - Tantor needs to catch up.
Tantor has one positive and that is it's format. It is mp3. It can be put on as many devices as you like. It is not proprietary. You can listen to it through your music player on an iPad or an android device. You do not need to use the Tantor app. In fact, I would not recommend that Tantor app at this time. It is a mess and needs a lot of work. The Tantor website needs work too. Tantor has been testing a Beta site which is more functional but it is not up and running yet.
Tantor does not have a membership available like Audible or Downpour. The prices are not as expensive as Audible but then again the catalog is very limited. For example I tried searching for one book listed in all three vendors to compare prices. Tantor did not have any of the books that I could find in Audible or Downpour. You also have to be very careful with the listing at Tantor. Some listing will have the download version (as opposed to the physical cd version) as Audible. You need to make sure you are selecting the mp3 download and not the Audible download, which will be more expensive and proprietary. I am not sure if Tantor is owned by Audible/Amazon or just in a partnership of some type.
One last plus for Tantor is the staff. Anytime I have had a problem with them, it has been resolved relatively quick with one or two emails (number of emails is dependent on how coherent I am, not their fault). I think Tantor can really be a great option if they get their Beta site up, fix their app (no on second thought, throw the app out and make a new one from scratch) and increase their catalog. If they are not going to compete head to head in sheer number of titles, they can find a niche for themselves. Tantor just needs to decide what they want to be and make it happen.
Downpour http://www.downpour.com/ The best audio book option for the money.
Downpour's membership is $12.99 a month, almost two dollars cheaper than Audible. You can buy additional "credits" for $12.99 at any time. Almost all of their books are only one credit. Occasionally they have sales where you can purchase additional credits for a lower price. Their books are reasonable without the membership. They have excellent sales. I suggest signing up for their newsletter or following them in social media to be notified of the sales. Downpour's titles are in mp3 format and are DRM-Free. You can put it on any device and not be limited by how many devices you use. There is a huge catalog to pick from. Anytime I have had questions or issues the Downpour staff has been very helpful.
Downpour prices are much better than Amazon. To illustrate the price difference I found the exact same title in both vendors:
The Fellowship of the Ring: Book One in The Lord of the Rings Trilogy
Written by: J. R. R. Tolkien
Narrated by: Rob Inglis
Length: 19 hrs and 11 mins
Series: The Lord of the Rings, Book 1
Audible price $38.49 Downpour price $ 23.50 (this title is not available through Tantor)
Finally I would suggest getting a title first from your library to ensure you like the narration before spending money on a title. Depending on the length of the sample provided at the vendor's websites, it is not always possible to tell if the narration will work for you. As in all things, when shopping for audio books, compare prices. Make sure you are comparing the same exact title, same length and narrator. If you go just by price you may end up with an abridged version.
A quick recommendation on narrators that you cannot go wrong with: the late Robin Sachs, Simon Vance, Ron Perlman, Richard E. Grant, Paul Michael, Avery Brooks, and David Warner, just to name a few.