the 570 because you can use the Mercury playback Engine in GPU accelerated mode. That´s only possible with Nvidia cards and not ATI.
It helps to speed up rendering of some effects, although not all effects use it.
with the 570 I think you´ll have to modify a .txt file in order to have it activated/supported, but it´s really easy to do!
You can read more about it here,
http://www.pugetsystems.com/labs/articles/Adobe-Premiere-Pro-CS6-GPU-Acceleration-162/
---EDIT---
hmm, seems I was abit quick there, it seems that only works on windows, not mac... for mac the ATI card might be a better choice! read the link I sent and you´ll get all the info you´ll need.