The polar distortion doesn't really matter: at the moment, even if you make a properly distorted texture, it will look like a mess at the poles. US3 might be more accurate at high latitudes, though.
Just a general GIMP note:
I see a lot of use of the solid noise / difference clouds filter. While this filter works fine for making bands of clouds for gas giants, with terrestrial planets the "plasma" filter usually works better, as long as you make the plasma seamless.