- Flash is better for cel-shading and it's easier to color if you're more of a cartoonist.
- Photoshop is better for smoother coloring/shading and not as great in cartoon drawings as flash.
- Illustrator is pretty much something you use for creating stuff you're planning on printing on posters, shirts and whatnot.
- I am not very familiar with Gimp, but it's like a much simpler photoshop
- Go to some oekaki that has ShiPainter and try it out. It's simple and easy to jump into, and knowing that it's an oekaki gives you the feeling that you don't really have to put that much effort to make it look good. It's generally just for sketching.
So what exactly are you going for? What type of drawing style would you like to try? Type of coloring and shading?