1. Key Difference 1: Features - MacDown has a few additional features that Mou does not have, such as the ability to automatically scroll to the current line, and a live preview feature that updates the preview as you type. Mou, on the other hand, offers a built-in HTML export feature, which allows you to export your Markdown documents as HTML files with a single click.
2. Key Difference 2: Customization - MacDown provides more customization options compared to Mou. MacDown allows users to customize the editor's theme, font, font size, and also offers support for multiple color schemes. Mou, on the other hand, has a more limited set of customization options, with only a few built-in themes available to choose from.
3. Key Difference 3: Active Development - MacDown is actively being developed and receives regular updates, adding new features and fixing any bugs or issues. On the other hand, Mou has not seen any updates since 2016 and is considered to be no longer actively maintained. This can pose potential compatibility issues with newer operating systems or lack of support for future enhancements.
4. Key Difference 4: User Interface - MacDown has a clean and minimalistic user interface, with a sidebar that displays an outline of the document's headings for easy navigation. It also provides a Table of Contents feature, allowing users to quickly jump to different sections of the document. Mou, on the other hand, follows a more traditional two-pane interface, with the editor on the left and the preview on the right.
5. Key Difference 5: Price - MacDown is free and open-source, while Mou is a paid application available on the Mac App Store. This difference in pricing may be a factor to consider for users looking for a free solution or those who prefer a paid application with dedicated support from the developers.
6. Key Difference 6: Integration - MacDown integrates well with other Markdown-based platforms and tools, such as GitHub and Jekyll, making it easier to seamlessly work with these systems. Mou, on the other hand, does not have the same level of integration with external platforms and tools, limiting its usability in certain workflows.

In Summary, MacDown offers additional features, more customization options, active development, a cleaner user interface with a table of contents feature, it is free and open-source, and integrates well with other Markdown-based platforms and tools compared to Mou.