The question of which is better, Gimp or Photoshop, basically revolves around whether you’re a graphic designer or someone who just needs to get a job done. Apple/Macintosh and Windows users war with Open Source purists over issues like these, and Adobe experts duel with oftentimes frustrated Linux users. Things like this can easily get ugly, especially since Adobe has not seen fit to port their high end graphics programs to Linux.
For the sake of clarity, let's look at some of the attributes and liabilities each of these programs bring to the table with them.
Photoshop wins the feature battle hands down. It's an older, far more mature project with a huge staff of capable programmers. That said, ordinary users need only a tiny fraction of what either Photoshop or Gimp are capable of doing. If you want to create a great looking logo or some icons or dress up a web page, Gimp is more than adequate. If you work in a graphic production environment for print publications, you can forget Gimp. It doesn’t have the kind of CMYK+ support print people need, and its font handling is atrocious compared to Photoshop.
Both programs are difficult for a complete novice to learn. Even things that would seem simple, like drawing a rectangle, require way more insider’s knowledge than most users bring with them. While Gimp’s UI is quirky but somehow still productive, Photoshop’s is far more polished with loads more fit and finish.
Adobe’s Photoshop documentation is well integrated, comprehensive, and feels like the work of a competent team that put hundreds of man-years into the job. Gimp’s documentation is hard to find and comes as a separate download. It’s much lower rent-looking and not at all pretty, but it’s thorough and will help you get the job done once you can find it.
There are a couple of good books on Gimp. No bad ones that I’ve seen. There are literally hundreds of books on Photoshop, many of them of very high quality. If you think of websites as documentation there are thousands devoted to each one, but Photoshop still wins hands down. Gimp folk will tell you about Gimpshop, which makes Gimp look a lot like Photoshop.
Gimp is free. Photoshop is comparatively expensive, though given its feature set, Photoshop is amazingly cheap.
Gimp support is haphazard. They don’t have a central support team, like Adobe. But it’s free, and if you’re persistent you will most certainly find the answer on a forum somewhere without paying a cent. On the other hand, Adobe support is expensive but has higher availability. Third party support for Photoshop is a universe of its own, with everything from your local PC configurator guy’s girlfriend to straight off the Adobe Classroom certification.
Photoshop isn’t copy protected, but you do have to register it. Adobe is generous about letting you run on both a desktop and a laptop, for example, and they haven’t balked even though I go through computers suspiciously fast (nothing sinister, they just deteriorate quickly in my house). Still… Gimp is free and certainly doesn’t require registration.
Photoshop is much nicer to use than Gimp. It's highly polished and clearly at the head of the class. Gimp is still full of rough edges, for example its difficult to use text tool.
Programming Gimp is not for the fainthearted. Photoshop has a much more comprehensive automation model. You can do lots of awesome stuff with Gimp nonetheless. Much of its default functionality is in the form of plugins built with Gimp’s unusual Script-Fu language. You can hack Gimp in Python, but that means a less unified experience.
All graphic artists know Photoshop. They have to. It pays the bills. Only a tiny fraction of professionals know and use Gimp. If you want to find good artists cheap, stick to Photoshop.
Photoshop is measurably better than Gimp in every significant way. To me the most persuasive reasons to go with Photoshop are the availability of good artists and the greatly increased likelihood that in case of emergency, you will do much better finding a solution with Photoshop due to the vast third party landscape.