The single fan item is a reference design meaning it runs at stock gpu and memory speeds. It also uses a cooling solution that is not intended to cool the card when overclocked by very much. It also will be nosier at higher fan speeds required for an over clock. There are 2 versions of the duel fan card and I am not sure which one you are showing. But the top duel fan card is factory overclocked both memory and gpu speed. The other duel fan card, I would expect, can be over clocked as it is set to reference speeds but has the cooling solution from the faster card. Also usually a duel fan set up will be quieter under demanding game use.
Its all based on your personal preference. Either would be good. I did not want to wait for cards to get into stock so I went with a HIS 7970 and I love the card. Hardocp/guru3d and toms have pretty good benchamrks and tests. All will only OC to a certain speed and youtube 7970 review. Motherboards.org and guru3d have some pretty comprehensive benchs for different 7970s with their heat, power consumption and fan noise. hopefully that helps.
I ordered an XFX DD black edition last week. I really didn't have much choice, the 7970s would appear on tigerdirect and then sell out in a couple of hours. Newegg has had the same two available for some time which makes me wonder why they haven't sold out of those particular brands.