One major factor in whether you should use Qt or GTK will be your programming experience: if you are already familiar with one, that will make your ArcObjects experience easier. In addition, each has both benefits and disadvantages.
Both Qt and GTK provide benefits of multi-platform availability, widespread use in newer projects, ease-of-use for programmers, and active development. Otherwise, pros and cons for each include:
Pros
|
GTK
|
Qt
|
|
flexible L-GPL license
|
available commercial support
|
|
|
cross-platform gui-builder
|
Cons
|
GTK
|
Qt
|
|
non-centralized support
|
less flexible and/or costly licensing
|
|
documentation is lacking
|
|
|
more difficult to set up for Windows
|
|
|
no mature gui-builder
|
|