Both tools turn content into designed ebook PDFs. They divide the work
differently. Designrr gives you a WYSIWYG page editor: you pick a
template and arrange each page, which is why it also reaches ePub,
Kindle and flipbook output. Unplain keeps the page layout automatic and
gives you an editor for the content instead. You click any text to
change it, insert headings, lists, images and buttons, and Unplain
handles the cover, contents, pagination and branding around what you
wrote.
The short version: with Designrr you own the page, with Unplain you own
the content. That is the trade, and which side is better depends
entirely on whether page-level control is worth the time it costs you.