Photoshop. It is better documented and better supported by on line communities. In the end, Photoshop remains the gold standard.
GIMP also has one HUGE gaping hole, although the upcoming GIMP 2.4 will help close this at least a bit - GIMP does not support color management. For anyone who is serious about image editing, this is a kiss of death. This means that GIMP is good for casual editing of casual photographs, or any other material that is destined exclusively for the web, where sRGB is the right color space. For anything else, you need the golden standard, Photoshop.