If it was a car question, you could ask "I'm driving to Timbuktu, should I drive a Ford, or a Chevy"?
Pinnacle Studio will not be putting any heavy demands on your video card.
If this is the sole purpose of the machine, either one will do a great job.
I have used both nVidia and ATI cards. The ATI cards just look better.
The nVidia cards are super-fast for the most demanding 3D games out there,
and ATI/AMD is giving them a really decent run for their money.
MY vote is for ATI since a quality picture for your application is most important.