I haven't had any significant performance issues on either DE myself. KDE seems to take a little more time to load and shutdown but feels slightly faster when browsing files. However, I couldn't detect any appreciable speed difference on the applications I use regularly (mainly Opera, Firefox, OpenOffice and a couple of IDEs).
No significant difference in core functionality, either. I was able to listen to music, browse the web, write code, burn cds and generally do what I wanted to do without issue. Both DEs pretty much offer the same set of operations with slightly different looks and details - there's no "paradigm shift" to speak of, when moving from KDE to Gnome or vice versa. (Unlike, for example, when moving from KDE to Mac OS X, which entails a shift from a window-centric environment to a document-centric environment).
In any case, I consider both KDE 4.3 and Gnome 2.28 as decent choices for a DE. As far as I'm concerned, there's no clear winner, there's no terribly compelling reason to switch from one to the other - at least not right now (you can never know what the future holds).