quoting Nortaneous:
1) Packages are much more likely to be up-to-date in pacman. Ubuntu and Debian are notorious for leaving very old versions of things in their repos; a lot of programs tell you to manually install things from their own sites instead of getting them from the aptitude repos, which totally goes against the point of having a package manager.
This. So much this. I mean, what's the point of having a package system if you're not going to be running the right version of things...
But I do like KDE. I can't say I've had that much difficulty with Kubuntu... most of my problems were my own idiocy.