The popular story, as I've always heard it, is that the PC-9 was the preferred military option, but as the Tucano was to have been built in Belfast it was the preferred political option. Jobs came to Belfast and so too did the opportunities for the IRA to bomb the production line, and at least one Tucano was destroyed at the factory.
The T-6 is a totally different aircraft to the PC-9; think Harrier II to tin wing Harrier.