Obtainium was also mentioned on the Reddit thread: https://www.reddit.com/r/degoogle/comments/yq8y6a/track_android_apps_releases_directly_from_the_repo/
This was my very quick, highly assumptive comparison between the 2:
Good question. I hadn't heard of Obtainium before, thanks for sharing. They do appear to be similar. Making some assumptions here. Just looking at their GitHub I would say the differences are:
Obtanium installs apps for you and requires permissions to install apps on the device. My app does not install the apps for you and does not require this permission, it will send you to the webpage where you can download the apk yourself. 'Install app' permission could then be given to your web browser or file browser and not to me.
Obtainium also needs other additional permissions such as to run on startup and alarm manager (to poll for updates?). My app requires you to open it to check for updates and currently does not run on a schedule or send notifications.
My app requires the 'query installed packages' permission to determine which version (if any) you have installed. Because Obtanium doesn't have this permission I'm assuming that they're tracking those details internally to their app, meaning that if you install the application direct from the website Obtanium won't be able to see the it.
Obtanium says that they use web scraping for some data which may break with webpage changes. My app uses GitHub and GitLab APIs for repo information, and files such as build.gradle to determine package name. These are less likely to change over time. If the project is not a standard Android project structure then my application may not be able to determine the packageName.
Obtanium is a more feature rich application. More apps sources. Import/Export. My app doesn't even have any settings yet (blank page).
Obtanium is written in Flutter. My app is standard Android+Kotlin.
Summary - Obtanium is trying to be closer to a feature rich alternative app store. My app is closer to the minimal RSS Tracker.