Both were difficult to use at start from my experience with 'em years ago, but after time, they became easier to use. If you have the patience to read tutorials and follow steps, you should remember your way around the programs eventually.
As for the question, I prefer PhotoShop. I'm not sure why, but every time I saved an image in GIMP, it lost some of its quality. Layers, animation, cropping and making backgrounds transparent are a lot easier in PhotoShop too.