Having used a Kindle for a while, I'd recommend it for the simple reason that when you register it with amazon, you get an e-mail address that you can send ebooks to which are then automatically transferred to the Kindle.
So if your mother has a wifi kindle then you can manage it by converting books using calibre then sending the file to
mother@kindle.com (or whatever e-mail is created for her) and it will automatically appear on her kindle whenever she is in range of the wifi or plugs the USB cable in.
All you need to do during your next visit is to delete the books that are no longer needed and theres no requirement for your mum to ever need to understand transferring files or converting books.