GTK:
+ life is easier on Fremantle with GTK as this is the native toolkit with full support for everything
+ can write code in any language
+ cross-platform (Linux, Unix, Windows, Mac)
- community maintained documentation not always easy to read or complete
- GTK will be officially dropped for Harmattan and will only be community-supported (like Qt is now)
- some say Qt is faster than GTK
- object-oriented GObject C is something you have to get used to, but with higher languages, this isn't required

Qt:
+ will be the native toolkit on Harmattan
+ Qt apps written today are supposed to work mostly on Harmattan
+ code-compatible with Symbian in the near future
+ good documentation
+ cross-platform (Linux, Unix, Windows, Symbian)
- cannot write code in C, you need at least C++
- C++ code is not real C++, but has to be preprocessed by a special preprocessor for compiling