GIMP is completely free and open source, meaning you can use GIMP and all of its features without spending a penny. This makes it an excellent case for artists or designers who may not have the budget to spend over 700 USD on Photoshop. It is also available for free on Windows, Mac OS and Linux. I...
Gimp has less features than programs like Photoshop, but for a beginner, or someone not needing complicated options, Gimp is the perfect choice. It allows you to quickly do basic photo manipulation so you can easily get the product you need without having to worry about complicated features getting
Krita's preset brushes are one of the default dockers. Each brush has a preview on mouseover that shows a detailed view of the type of brush involved. All the brushes also have useful and descriptive names such as "HP Pencil" or "Textured Fuzzy".
Krita's official documentation is incomplete in some areas, especially for new features that are constantly added. But this is compensated with it's great design and usability which makes it easier to understand how things work.
Since releases are so often and with little time between them, usually new features are not tested a lot and this can bring a lot of bugs with them. Which fortunately are quickly patched in the next release.