it doesn't really matter. the only thing that is different between each 7970 is the cooling solution. get the cooling solution that appears most appropriate for your setup.
you may also consider getting a factory overclocked edition, as they typically choose the chips that came out of the manufacturing process better than the others to overclock. so in theory they are better.
you may also consider getting a factory overclocked edition, as they typically choose the chips that came out of the manufacturing process better than the others to overclock. so in theory they are better.