is there a significant difference between Aab vs apk file for users?
Yes: users can not install AAB files directly.
You can read more information about Android App Bundles in the documentation, but the gist is that AAB are intended for publishing to Google Play where APK are for installation to the devices. In fact, Google Play generates APK files from the AAB when the user installs a new application from there. Google Play requires AAB because it allows them to generate smaller APKs based on the user’s target device.
You also can generate APK files from an AAB with Google’s bundletool, therefore you could install an AAB to you device, if you wanted, but it is cumbersome for most users.