[gsliepen]

I used GIMP before I ever used Photoshop. My experience was the opposite. I think that means the UIs are different, but there is no one that is objectively better, it's just a matter of what your expectations are, which are set by whatever you learned first.

As for CMYK support: why do designers even need to use this? Sure, not every RGB is the same, and it took some while before we even got sRGB as some standard, but the same goes for CMYK: every printer has its own profile. I had the displeasure of trying to get the CMYK profile of a "professional" printing company that only accepted files in CMYK, and they didn't even know which profile their printers used. Ideally you would send a RGB file including the display profile your screen uses, and then the printing facility converts that to whatever CMYK they need.

Of course there are also special colors or effects outside of RGB/CMYK that you might want to use when printing something, that's something else.