When comparing Pixelmator vs Paint.NET, the Slant community recommends Paint.NET for most people. In the question“What are the best pixel art / sprite editors?” Paint.NET is ranked 9th while Pixelmator is ranked 11th. The most important reason people chose Paint.NET is:
Unlike most photo editing software, Paint.NET is simple to learn and easy to use. This means less time is spent learning the in's and out's of Paint.NET and more time on photo editing. For those who only edit photos sporadically and don't have time to invest in learning complicated tools for editing, Paint.Net will suit your needs.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Beautiful, easy to use UI
Pixelmator's interface is pleasant to look at, easy to use and specifically optimized for OS X users, making it very intuitive for long-time Mac users.
Pro Feature rich
Not only does Piexelmator provide easy to use tools that you should expect in any image editor, it also has rare tools such as layer styles, liquify tools and OS X optimized, fast image manipulation engine among others.
Pro Supports layer styles
Layer styles allow non-destructively applying effects to a layer.
Pro Powerful effects
Dozens of visual effects available, easily discovered from the visual Effects Browser, broken into categories. Effects have interactive dialogs and live preview.
Pro Multiple-display support
Pixelmator can be used across any number of monitors.
Pro Loads quickly
Gives you common image editing capabilities almost instantly.
Pro Great performance
Pixelmator's image engine is specifically optimized for use with Macs, providing one the best performance results available on OS X.
Pro Inexpensive
Pixelmator costs just $30 on the App Store. Additionally, it often comes bundled with other software that you might also find useful bringing the price down.
Pro Easy to learn and use
Unlike most photo editing software, Paint.NET is simple to learn and easy to use. This means less time is spent learning the in's and out's of Paint.NET and more time on photo editing. For those who only edit photos sporadically and don't have time to invest in learning complicated tools for editing, Paint.Net will suit your needs.
Pro Free software
There is no charge for this software; but donations are welcomed.

Pro Lightweight
Paint.net provides a streamlined but effective core tool set. Support for advanced and niche features is provided through modular plugins, allowing the user to install only what's needed.
Pro Actively developed
The project is active and being continually expanded and refined.
Pro Active and friendly community
The forums are filled with tutorials, as well as people who are friendly and eager to help.


Pro Plugins allow extra functionality
Plugins can be made by anyone in the community, and are offered for download on the forum. They extend the capabilities of Paint.NET to cover most tasks.
Pro HiDPI support
Paint.NET supports fully HiDPI displays, with no scaling problems.
Cons

Con Only available on Windows
Paint.NET is only available on Windows.
Con No project-based interface
You can only edit a single image at a time in Paint.NET. For example, in other tools you can open 5 or a 100 images at the same time, and stack those windows in all directions.
Furthermore, when you are editing in Paint.NET you can only "view" a single image at a time, even though you can have multiple images open. This limits your zooming ability, as the image will go fullscreen quite quickly, and hide parts of it behind the floating toolbars which can not be embedded anywhere on the screen.
