Photoshop as always. when it comes to soft-brush drawings that's what i always use. but for the fuzzy strokes you need a tablet and then set the brush for "soft" and use the "smear/smudge" tool. the smear drags the color into a spike and the "soft pen" makes it fuzzy and i give it even more "fuzz" in some places with the "blur" tool. and for each color i use 3 shades. main, light, and dark. i start with the main color and i blend the other 2 in to make dark shadows an light highlights. oh and i also like to use a diamond-shaped brush instead of circle it gives more pointy spikes. GIMP (that you have) can do all this with a tablet.