wxPython, and I'm assuming PyGTK also, can use wxGlade to help you design most UIs you will create. That is a big plus. You don't have to learn how to hand-code the GUI until you're ready. I made several GUI programs straight from wxGlade before I was comfortable enough in how wxPython worked to take a shot at hand-coding.
PyQt has a similar graphic layout device but I've never had good luck getting PyQt to compile correctly. There was also a lack of tutorials and documentation that showed how to create the final Python code; many of the documents I found referred to the C++ version of Qt.
Tkinter is good for quick and dirty programs but, realistically, if you use wxGlade it may be faster to make the program with wxPython. At a minimum, you can use wxGlade to show a visual representation of the program to a client rather than take the time to hand-code the "dummy" program.
