Well i wrote this up and then went and reread what was posted xD have it anyways
To be honest a decent program will take a while to learn; SAI, photoshop, and gimp are all powerful tools but in terms of learning curve they are about the same. It all comes down to how much time you put into each program. Though personally I would say photoshop has the lowest learning curve to two reasons, one its a product released by a corporation so you will have less "runtime" problems and two it is more prominent so you will see more tutorials out for it.
Inkscape also happens to be a vectoring app
and i can confirm that the Mac port of SAI is horrible.
EDIT: Grammer