Both are perfectly good alternatives to listening to Audible audiobooks, but they have slightly unique features that set them apart from each other.
audiobook shelf
Audiobookshelf is a lot like a Plex server, but for audiobooks and podcasts. You can upload your audiobook files and it will create a library with covers, metadata, chapters, and listening progress for you.
You can listen to your audiobooks in a web browser or in the AudiobookShelf apps for Android and iOS, which support offline downloads. It also syncs your progress between devices, allowing you to continue in exactly the same place if you switch from one to another.
If you have a large number of DRM-free audiobooks on a hard drive, go ahead and start here.
Storyteller
Storyteller also offers basic audiobook features, but it’s really designed for those who often alternate between reading a book and listening to it. If you upload a DRM-free ebook and a DRM-free audiobook of the same title, Storyteller will sync your progress between the two as you go.
I use this feature almost every day between listening to an audiobook in my car and then switching to my e-reader to continue reading at home. Storyteller effortlessly saves my place in both using EPUB 3 media overlays. And like Amazon’s Whispersync, it can also highlight the current line of text while the audiobook plays.