both are about the same in terms of raw power with the 7970 being a bit faster. Problem is that AMDs drivers are trash.
Get a gtx 670
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127685
this one will overclock like a demon and stay cool
Id stay away from reference models unless you are down to take them apart and really TIM or get a new cooler for them.
I got a used zotac gtx 670 for 270 and did just that now it runs and ocs like a champ.
Recently I have played around with a 7950 and 7970 and let me tell you that the drivers are putrid imo...
sure they have raw power but inconsistent frame delivery and a pathetic driver team, not worth it~!
All you have to do is google AMD frame latency to see what I am talking about. The latest (beta) drivers claim to take care of it but I doubt it!
Nvidia is a smoother more enjoyable overall gaming experience imo.