Well for one, koreader is awesome.

It has preferences I like, syncs reading position to anything else running koreader, and syncs my current reading queue from Dropbox.

Kill ads on the cheaper one would be my main if not only use case.

Installing an alternative reader that can read other formats is mentioned a few times, but honestly, with Calibre that became a non-issue decades ago. Need to transfer them via cable anyway, converting in the process takes a few seconds at most.