I use wxPython quite a bit for my GUI needs; the simplest reason for me is that I've found it much easier to distribute wxPython apps than PyQt apps. (Additionally, Traits are better supported under wxPython that Qt, but that is changing.) That being said, XRC and Boa Constructor and Glade (which can also build wxPython GUIs with wxGlade) are all way, way behind Qt designer in sophistication and reliability.
So, because I was willing to spend a bit of time writing the layout code, etc, and because I had to distribute to several platforms, I went with wxPython. But if you have a more controlled deployment environment and you want to design graphically, PyQt might be a better fit.