Well.. one is Free and one cost over $500... so this really isn't a fair comparison. Most users agree that The Gimp is great given it's price.. for me, I couldn't get used to the interface and the install left a few things to be desired when I tried it.

Photoshop is the best of the best... few programs come close to its universal appeal to professionals and the shear amount of plugins/support for it.. it is without a doubt one of the best.