What should I choose: GTK+ or Qt?

As you seem to primarily target Linux, then the choice mostly depends on the programming language you want to use.

If you code in C, then obviously go for GTK+

If you code in C++, go for Qt, otherwise you will need Gtkmm (a C++ wrapper over GTK+)

If you code in Python, both GTK+ and Qt have bindings for the language: see PyGtk, PyQt and PySide (the one launched by Nokia themselves).

If you code in Java, Qt is no more a viable option imho as Nokia discontinued Qt Jambi (the Java bindings for Qt).

Also, Qt is more top-notch regarding its scenegraph QGraphicsScene API, its scripting engine built over Javascript Core (the engine powering WebKit), its state machine and animations framework, and the declarative UI.

GTK+ doesn't offer that much although you can use Clutter alongside with it.

If you're specifically looking into DB, XML (GTK+ has a parser for a subset of XML) and threading (GTK+ has GLib) features then Qt will offer all that in QtSql, QtXml and QtConcurrent.

All in all, I would say Qt is a sure choice. But GTK+ is very capable as well.

I'm not sure you will get a crystal clear answer for your question, which explains why some people keep preferring Gnome over KDE or vice-versa. Choose what works best for you.

PS: I you plan to also target Symbian, then go for Qt.

EDIT: Something that is also great with Qt is QtWebView: it brings Chromium into your Qt application to display web content. Others are embedding web content into their application using for instance Awesomium or Berkelium.

I've used GTK+, QT and wxWidgets before. Here's a brief summary:

For my first cross platform UI project I decided to go for wxWidgets mainly because at the time the license wasn't as restrictive as QT's (QT was GPL and only for Linux) and it had platform specific UI (unlike GTK). The project worked out well but there were quite a few glitches getting it to compile and run properly in other platforms - sometimes some events were fired up differently and such. Also GDI in wxWidgets was pretty slow.

Next I used GTK for a different project in python. For this I used the python bindings and everything worked out more or less smoothly. I didn't quite like the fact that the UI didn't look native on Windows and Mac and also when you launch a GTK+ app it always debug outputs loads of CRITICAL warnings which seem fine to ignore. :S

Finally, I did a very simple QT project now that Nokia has acquired it and was brilliant. The best of the three. First off, if you're not an old schooler who prefers VI or Emacs, QtCreator is brilliant. I really love VI and used it for years but I much prefer QtCreator for C++ QT projects. Regarding the library I also liked a lot the documentation and the APIs provided. QT has a concept of slots and signals which introduce new C++ keywords and a preprocessor. Basically, after reading a tutorial you'll get it easily and will start to love it. I'm now doing iPhone dev and it does feel a bit like Cocoa's/Interface Builder's UI paradigm.

Summary: I'd go for QT hands down. The license is pretty good and the SDK and documentation really nice.