Definitely faster vram. The speed of a vc's ram directly affects how fast it is by increasing it's band with, while all a bigger quantity of ram will help you with is coping with higher resolutions and more antialiasing better.
The 6570, however, is a vc that's designed for low resolution gaming, so 512mb will be more than enough...
Even if run a high resolution with it, you'll be forced to tone down on the visual settings a lot, which lessens the amount of ram you need for the respective resolution, so you won't need more than 512mb anyway.