[zokier]

http://img9.imageshack.us/img9/6628/smallphotoshop.png

Yes, its 100px wider, but still, imho Photoshop makes a lot better use of precious screen space. Compare the buttons under layer palettes. GIMP has 32px icons, while Photoshop has over 10px smaller ones. Or the slider, in Photoshop under "Navigator" and in GIMP in the Layers palette. Actually just compare Layers palettes in each. GIMP uses lots of superfluous labels and huge font size compared to Photoshop In my screenshot Photoshop also has 2 more palettes on screen.

Photoshops UI is also quite flexible, allowing to move and resize palettes freely, dock and undock, and group palettes to tab-groups. And layouts can be saved and then selected with just few mouse clicks. This allows it to scale across monitor size and count relatively easily.

edit: Sorry for inflammatory Photoshop vs. GIMP post