Krita 5.3/6.0 is the result of many years of work by the Krita developers. Some features have been rewritten from the ground up, others make their first appearance.
Enjoy the completely new text feature: on canvas editing, full opentype support, text flowing into shapes. It is now easier than ever to create vector-based panels for comic pages. Tools got extended: for instance, the fill tool now can close gaps. The liquify mode of the transform tool is much faster. There are new filters: a propagate colors filter and a reset transparent filter. Support for HDR painting has been improved. The recorder docker can now work in real time. There is improved support for file formats, like support for text objects in PSD files. And much, much, much more!
Depending which version of Qt and KDE Frameworks you build, the same source will result in one of the other. Both versions are almost functionally identical, with 6.0.0 having more Wayland functionality. But note that since Krita 6 is still considered rather experimental.