Hi,
Well, it's not so much off topic as it seems. These days I tend to prefer GTK3. Even though they changed quite a bit and it takes getting use to, benefits you get from adapting are worth it. Perhaps I find it easier to customize due to CSS use in GTK3 styles and my experience with web development. In case of Sunflower, all the custom drawing code I did in GTK2 was removed and replaced with a simple CSS class. Code became simpler and therefore easier to maintain. Not only that, all the complications with colors were also resolved easily.
In addition to that, GTK3 in Python is used through GObject Introspection, which allows direct use of C library without any middle layer such as PyGTK. This essentially keeps the feature parity with C library and gives me a lot more flexibility. There are some downsides to this approach. Some constructors don't allow constructing objects by calling class constructor, instead they expect us to call class method to construct a new object. The problem arises when interface crashes and you don't know why since problem occurred in library. So now you have to use gdb to figure out why your software crashed. This is not a big of an issue as it happens rarely. Mainly a problem after switching from PyGTK and getting use to new way of handling things.
As for Qt, I can't really comment. I only know few basic things. It looks like a decent toolkit. My goal originally, with Sunflower, was to provide a twin-panel file manager for Gnome environment as KDE is spoiled for choice there. So GTK was the only way to go. One might argue that Qt has better cross-platform support but it seems that might not be that big of a difference considering that GTK3 has support for different rendering backends. For example here's screen shot of Sunflower running as web application using Broadway GTK3 backend (
https://i.imgur.com/b3Tvcs5.png). You can easily see the appeal of managing your files or starting applications from afar. Only difference being that GTK widgets would look out of place on other platforms.
Speaking of widgets, I really like what GTK folks did with new version. Even though sometimes it might feel like they are still searching for the final set of widgets (features being added and deprecated frequently) it's still a flexible toolkit. Popovers are one thing I really like. We use to have clunky menus which could hold any widget but you didn't know how theme would handle drawing and you had to take into account a lot of other weird issues with focus. Popovers are a simple and elegant solution to that. They behave like menus but allow you to have multi-layer structure with different widgets. New application bar is also nice. Allows for some clean designs while giving developers more flexibility.
Sorry for long reply!
tldr: GTK3, and yes I would use it again. :)
All the best!