Well, I have worked with both for a little bit of time, and this is my opinion.
If your environment is "project centered" and you need to track activities and assign a variety of Outlook features to a project (E-mail, task, appointment, journal note, etc.) Then Kratz's method is more powerful, and can be used without any "modifications" to Outlook. I used it with Agendus and could also quickly create a project history on my Palm.
However, if your work environment is E-mail centered, and your tasks, delegations and appointments are generated by E-mail and communicated to others via E-mail, then the GTD Add-in is easier and quicker. At least, at this point this is the case, in my view. NetCentrics is working on improvements to the Add-in that will make it more project friendly, as I understand it.
HTH,
Gordon - in Canada