When comparing Notion vs BookStack, the Slant community recommends BookStack for most people. In the question“What is the best multi-user wiki?” BookStack is ranked 6th while Notion is ranked 7th. The most important reason people chose BookStack is:
The stock interface design has a modern feel and is simple for new users to use.
Specs
Question Ranking
Pros
Pro Does a great job with both notes and to-dos
To-dos in Notion aren't just dot point lists. You can drag and drop them into columns just like Trello (Kanban style), you can have sub-tasks, and you can easily mark things off as completed so they are no longer in your way. Notes are also powerful, with proper formatting and ways to manage and search for them, which makes it a great Evernote alternative.
Pro Flexible contents storage and organization
You can upload files and embed online stuff in any hierarchical structure using pages, toggle lists, etc.
Pro Awesome for wikis
You can easily start writing a bunch of web pages, share it with co-workers and choose whether to publicize or keep your contents private. It's also easy to hyperlink pages.
Pro Clean & simple default interface
The stock interface design has a modern feel and is simple for new users to use.
Pro Free and open source
Released under the MIT license.
Pro Multilingual
Over 9 different languages are built in to BookStack which can be set at a per-profile level.
Pro Full role and permissions system built in
Granular permissions can be set up for specific roles on a per-content basis and permissions will waterfall down to child content.
Pro Supports SSO using Google Apps
Very convenient when you're a company that uses Google Apps, that makes logging into Bookstack more secure and more convenient.
Pro Support for domain authentication
Can be deployed within an active directory environment with its features to connect over LDAP.
Cons
Con Designed for teams
As an Evernote alternative, this was mostly designed for individual use, so if you aren't a part of a team a lot of the functionality of Notion is pretty useless for you and might result in a more cluttered interface.
Con No support for readable markup languages
The content entered through the WYSIWIG editor is stored as HTML. The HTML can be edited directly, but no other markup language is parsed. Speak ReST, Markdown, etc.
Con Limited to 2 levels of content
Bookstack implements a "Book / Chapter / Page" system which works fine, but lacks flexibility when you need to have more levels of imbrication.
Con Limited customisability
Customisation options are limited with only the main theme color, logo and name being customisable.
Con Complicated installation
The current installation process involves many steps and may be a lot to take in for people not familiar with setting up Laravel applications.