Differences between Qt and GTK programs?

GTK and Qt are user interface toolkits and frameworks. GTK is the standard toolkit for GNOME/Ubuntu while Qt is used in KDE/Kubuntu. From the user's point of view I'd say it's just the looks that differ.

Aside from small differences in layout and appearance, the GTK version also has a quick search feature for shortcuts/abbreviations that QT doesn't have which allows you to use a hotkey like CTRL + SPACE to bring up a search box to quickly find abbreviations. This helps quite a bit when you have hundreds of abbreviations to try to remember.

Another difference is that, if you are on Ubuntu, you may not already have all the QT stuff installed. On Kubuntu, you may not have all the GTK stuff installed. Hence it may make the download much bigger.