I find Photoshop CS2 to be, in terms of functionality and usability, a much better application than The GIMP.
My opinion slightly biased as I've been using Photoshop for a far longer time (since I was like 8

), but I find that its layout makes more sense than GIMP's. Photoshop is also slightly more powerful (though both have features the other lacks). GIMP is a great program, but I find its layout to be rather impractical, and its learning curve is steep compared to that of Photoshop.
However, in terms of features versus money, GIMP wins hands down. Albeit hard to use, it is completely free in every sense of the word, which cannot be said for photoshop.
In Photoshop, finding a feature is more straightforward than in GIMP; APS lists its functions a format that makes more sense to me. If you want to edit an image quickly, my vote's for photoshop. But if you want a free application that's just as powerful, go with GIMP.
If you're thinking of which to install, I would tell you to get both. GIMP has some things that photoshop doesn't, and photoshop has some things GIMP lacks. The complement one another to create a very powerful suite; photoshop with its ease of use, and GIMP with huge array of power and freedom.