Here's the difference ...
Kubuntu uses the KDE desktop, but the OS is still Ubuntu. KDE can be configured to look pretty much like Gnome did before Unity on Ubuntu. KDE has a lot more areas for configuration than Gnome depending how far one wants to go. I've always like KDE a lot.
Here's an interesting site with lots of information on various Linux distributions if you've not see it before. http://distrowatch.com/table.php?distribution=kubuntu
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