Haven't used either, but people typically say that Qt's API is cleaner, that it's better suited for cross-platform applications (for example it has support for

Haven't used either, but people typically say that Qt's API is cleaner, that it's better suited for cross-platform applications (for example it has support for Android) and that it's more flexible (for example LXDE is being rewritten as LXQt, since the devs found GTK3 too heavyweight and too GNOME-centric).