I've found QT in general to be a much more powerful toolkit
it's kind of everything but the kitchen sink or whatever the saying is. It has support for everything from OpenGL to QString (which actually I find pointless and annoying ) but they do provide APIs to do a lot of things really easily. My brief interaction with GTK is wayy dated, but I found them trying to be too close to the C roots which are very handicapped when writing a GUI application. OOP is much more easier to workwith and than GTK.
Biased obviously and it doesn't hurt that QT in general is kind pretty out of the box on all platforms.