Get whichever is cheaper, both cards will offer very weak gaming performance so that's almost not even a factor. Only use I would find in those cards is for extra display outputs, which can be handled by the cheaper GT210.
Actually, I've seen the 2GB 6570 go cheaper than the 1GB 6570. But yeah; the 6570 is an alright gaming card. The 6670 is a bit better; get it if it doesn't cost more than £10 than the 6570.
you can buy a GT 610 1gb i also have it and it can run crysis 2 perfectly though i have a pentium 4 which is not recommended on my board ITS A GOOD CARD , a mid range card