One is for digital painting and the other is, as it name says, an image manipulation program.
True. Usability, accessibility, and basic UI quality are all important focuses for the Krita devs.
Nevertheless, both are wonderful pieces of software! I'm not a graphics pro but do find myself editing images quite regularly, and I usually reach for GIMP as it's more familiar to me. Krita on the other hand has a great brush engine, so if I feel like drawing something, I go for that.
Also, of course, whichever one has GMIC on that particular day! Some of the stuff in that plugin is absolutely wild. Usually that means Krita again, though iirc at some point it was broken there for some time, but I found it for GIMP...
Btw, anyone know if there's a Rebelle Mixbox-style color mixing feature for either of the two?
I doubt there is anything that couldn't be done in either and it is simply a matter of time spent in the UI and personal preference.
GIMP is the one that's still missing proper colorspace handling, especially regarding CMYK.
And Krita has had vector layers with text support for quite a while as well.
But not proper native support for CMYK and spot colors as you might know from Adobe or Krita, where everything internally tracks CMYK data throughout.
It is much easier to make a photo collage in Inkscape than Gimp.