Tk is not very beautiful, but it's part of default install on many perl setups, which is a big advantage. Gtk needs a whopping lot of libraries to run (especially on windows I guess) though is better looking. I'd keep it simple and go with Tk.
Another interesting option is
which is beautiful, graphically very complete and waaaaay faster than Tk (it even supports OpenGl...) Oooops, I'm afraid the choice gets even harder now, sorry :)