you can check out anandtech and some other sites, but from my findings nvidia is kinda killing it with smoother multi card atm. performance wise the 7970s are taking the 670 by a few fps but depending on what 670 you get this isnt always the case.
i just picked up at an evga 670 ftw which is kinda taking the lead by a % or two. but i bought the 4gb version and you definitely pay for it, at $440 you can definitely get 7970s cheaper.
if you don't have an issue with the amd drivers the 7970s would be the better per $ recommend imo. Also with AMD cards you get a free copy of crysis 3 and bioshock infinite.. maybe you send one of each my way
personally we run AMD cards at work and i have enough hate for their driverset i popped for the nvidia even losing out on 2 free games, but its a personal choice