I can pretty much pick and choose whether I'm using the Windows or Linux build when I draw.

But in terms of performance (first) and stability (second), which build is superior?

Definitely not the most knowledgeable on this topic, but I think it can vary quite a bit depending on your hardware. In linux my AMD card wouldn't work with the openGL option on Krita for example.

Try again, I've ran into bugs with enabling OpenGL on both the linux and windows build and I have an nvidia/intel setup.

If you can use opengl, there's not much difference. If you can't, then Linux is better because on Windows Qt's painting system is slower than on X11.

Also, by default the Linux build includes support for OpenEXR and OpenColorIO, to get that on Windows, you need to get Krita Studio.

If you have a 32 bits Windows but a 64 bits Linux, go for Linux. The 32 bits build can use only about 1gb of memory for image data, and that's really very little.