Yeah the dual 6970s would be WAY more powerful, but you dont want to start a build with SLI/xfire cause you cant expand and then have 2 cards to sell. But if two of those cost the same as one 7970, go for the xfire system. (way better price/performance)
Or you can wait until the dual-gpu single card 7990. Dont know when thats gonna come around. Also Nvidia Geforce 600s are on the way.