Tk is old, ugly, and primitive. It's also well-supported and easy to run on any system. Wx is newer, looks prettier, and (I'm guessing) nicer to program with. However, I had problems getting it to work on my development system.

Not too long ago, I needed to make a GUI for a non-techie to import data from a local Access database and send it to our web server's database. Tk installed without a hitch, and Wx didn't. I didn't have time to mess with installing Wx, so I went with the "it works, use it" philosophy and did the project in Tk.