Which one and why?
People do have drivers issues with running a AMD card with and Intel system, and Geforce on AMD systems. If you have an AMD processor, go AMD card. The 7970 is a beast of a card, but most can run hot and that's not all that great for life. Mind you, you most likely will need a new card in 2 years or so. Plus the AMD drivers have been stepped up to help the compete with Geforce. Let me say that again, to help them compete. Geforce is working with some new techonolgies and drivers, i.e. the Geforce Experience, and are on track to just blow AMD out of the water by the end of Q1 2013 with drivers. The 7970 is roughly 50 USD cheaper when comparing the stock cards. The 7970, either edition, requires less power then the 680. However. the 680 uses the dynamic core that will reduce it's speeds and power draw when it's idle, using less power, and it can do super minor OCs to itself to give you that little power boost that might be needed some times. You asked why, that is why. My recommendation goes to Geforce because of how solid they are making the GTX 600 series. I have a 670 myself, and at full game load it never tops 60.
GTX 690 will always be if you have the money. XD That card rocks. It can't really be OCed though, but it's slightly better then two 680 for there is not scaling issues and detection issues.