Which one do you actually want?
Four questions. One of the possible answers is “neither, get a library card”, and we mean it.
Most people arrive at this subject with one of two wrong assumptions: that Audible and Kindle Unlimited are tiers of the same product, or that Amazon Prime already includes one of them. Neither is true. They are three separate subscriptions with three separate fees, and Amazon signing you into all of them with one account is what makes that so easy to miss.
Getting this wrong is expensive in a specific and avoidable way. People subscribe to Kindle Unlimited expecting audiobooks, find a narrow selection of narrated titles, and conclude the product is bad value — when in fact they bought the wrong one. People pay for Audible when what they wanted was a lot of reading cheaply. And a fair number pay for either while holding a library card that would have covered it.
The four questions below sort that out in about two minutes, in the order that matters. Answer them honestly rather than aspirationally: the reader you have been for the last six months is a much better guide than the one you intend to become.
Do you want to listen, or to read?
This settles more than anything else, because Audible and Kindle Unlimited are separate subscriptions doing separate jobs rather than two tiers of the same thing.
Not sure? Count the hours in a normal week where your hands are busy and your attention is free — driving, walking, gym, chores — and separately count the hours you sit down with nothing else demanding you. Whichever is bigger is your format. The long version is reading versus listening.
Have you tried your library?
Do this before you pay for anything. Most US public libraries lend both ebooks and audiobooks free through Libby. It costs nothing, we earn nothing from it, and for a lot of readers it turns out to cover everything they wanted.
The catch is waiting lists on popular titles and a catalog you do not control. If those are fine with you, you may be done here. See Audible versus Libby or Kindle Unlimited versus Libby.
Do the books you want actually exist there?
For Kindle Unlimited this is the decisive question. It is a defined catalog inside the Kindle Store, deep in independently published genre fiction and thin on mainstream traditionally published books. Search the next five titles you intend to read and count how many carry the badge — the method is here.
For Audible the catalog is broad, so the equivalent question is about the credit system: will you spend a credit every month on something expensive enough to be worth it? See credits explained.
Does the arithmetic work?
Divide the monthly fee by what one of your books costs to buy. That is how many you need to finish each month before the subscription starts saving you money rather than costing it. Finished, not started.
The calculators are on is Audible worth it and is Kindle Unlimited worth it. They run on your numbers, and they are perfectly willing to tell you no.
The three-way summary, if you only read one thing
Audible is an audiobook membership. You pay monthly; you get a catalog to listen to while you are a member, and on the higher plan a credit that buys one audiobook at any price, which stays yours. It suits people with real listening time and specific, often expensive titles in mind.
Kindle Unlimited is an ebook borrowing subscription. Up to twenty titles at once, no due dates, nothing becomes yours, everything goes back when you stop paying. It suits high-volume readers whose taste overlaps a catalog that is deep in independently published genre fiction and thin on mainstream traditionally published books.
A library card is free, covers both formats through Libby, and is strongest exactly where Kindle Unlimited is weakest. Its cost is patience: popular titles have waiting lists, and you cannot make a library stock something. It pays this site nothing, and it is still the first thing we tell people to try.
Notice that none of those three descriptions mentions which is “best”. That is not evasion — it is that the question does not have a general answer, only a personal one, which is why the four questions above are about you rather than about the products.