I think, the choice between MyPaint, Krita and ArtRage should depend on what and how you like to paint. And of course how much you want to pay for it. MyPaint, like ArtRage, has a very overseeable interface, while Krita has much more dialog panels and functions in the menu bar and the context bar. But MyPaint looks a little rustic and its brush engine is not as sophisticated as ArtRage's. ArtRage also has some features MyPaint doesn't offer, like e.g. stencils and stickers, a reference window (you can use PureRef or IrfanView for that, but it is not so convenient) and the GUI-Layout is comparatively rigid. Another Pro of ArtRage are the Tool Boxes, in which you can save all the colours, Tool Presets, Reference Images etc. you used in a document.

On the other hand, Krita has colour management and also supports CMYK. That is a big Pro. MyPaint and ArtRage haven't. And there are some filters and tools for image editing in Krita. It also has a Timeline for creating animations. But as far as I know, it doesn't support file types like *.gif or *.mp4, so that you need an additional software like VirtualDub to really make animations outoff it.

One thing is totally different in MyPaint and Krita on one side and ArtRage on the other: ArtRage has one brush for oils, one for watercolours, one Airbrush, one for pastels..., Krita and MyPaint have lots of brushes for each of it. And it is sometimes a matter of opinion what really looks like an oil brush, watercolour brush etc.

The easiest to learn is MyPaint. You will need a lot of time to learn Krita. Even Krita's layer system is comparatively complicated. But that's the price for the huge amount of opportunities. For me, Krita initially comes too overloaded, but fortunately it can be customized with so called "Workspaces", like also Photoshop and Affinity Photo have. It means that you can customize the GUI-Layout how you like and need it, save your Layouts and load them with just one click. So you can have different layouts for different needs. E.g. one for painting, one for animation, one for drawing, one for image editing... That makes the GUI much more overseeable and convenient.

MyPaint's development is very slow. The newest version is from 2011. In opposite to it, Krita's developers are very busy. Much more than the ArtRage developers, as it seems. ArtRage's development had the wrong priorities during the last years, in my opinion. But neither MyPaint, nor Krita offer these thick paint application and its interaction with the canvas textures and lighting. As far as I remember there was at least one brush in Krita that had a thick paint application. But it is not the same.

All three apps are nice in their own way, and you can paint great paintings with each of them. I personally still prefer ArtRage 6 for painting, but I have them all.

By the way, I also use Affinity Photo and GIMP. And you can paint with them too. But you can't blend colours into each other while you paint. You need the Smear Tool or the Colour Blend Tool (Affinity Photo) for it. So it is not like authentic painting.