They are pretty much the same in game performance with past to present games. Each card a little better than the other at certain things that probably wouldn't really matter much when comparing apples to oranges. Future proofing wise the 7970 has the edge with higher resolutions or playing on multiple screens because of the 3GB VRAM and 384 bit bus. AMD drivers might be a little flaky right now but will get better, not like ATI of the past. Honestly I've had more issues with Nvidia cards this year with drivers, and with that said let us move on before the fanboys kill me. =]

In short, I say get the card or brand you are most familiar with should you run into any warranty issues or driver configuration issues. EVGA is a good company and their card has a 3 year limited warranty, Sapphire is also good with a 2 year limited warranty. Both cards are massive so I hope your computer has enough space and the amps to power it. Good luck.