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 http://www.tkzinc.org 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 :)