Not sure about KDE, but Fedora's version of GNOME definitely works better than Ubuntu's version of GNOME. Because of their valiant efforts to keep Unity from changing at all for six years, Ubuntu's desktop has a peculiar hodgepodge of software versions and unnecessary patches. There's a sense of teams of developers pulling in opposite directions from each other with your computer straining in the middle. Hopefully will get better in a year or two. On the other hand, a lot of what's been going on around GNOME has been about how stuff fits together, along with backend changes that have been getting more and more polished in Fedora but aren't really there in Ubuntu yet. Kind of depends on your system (and whether you have strong opinions about search-as-you-type), but in my own experience I've found that Fedora just feels sturdier as a result. More designed, in a number of ways.

Also its boot splash is the coolest. That's probably the real reason.