Well, I don't use either, because I'm not really a photography person -- I just know it comes preinstalled on my computer. And that it's run by the same people who have recently made a huge leap in user-friendliness for linux, which I assume passed down to GIMP. It certainly doesn't require the command-line!
I am not really knowledgeable about this particular field, but I would think for most use it basically does the job, even if it requires you to relearn a new interface. But I wouldn't have a clue what kinds of special capabilities either have.