More games are being especially optimised for nVidia than for AMD.
AMD has historically have worse drivers and shoddy Cataclysm builds, but recently they have been quite good.
nVidia has, ironically, been having driver issues lately, and for me, I've always had issues with them. (but since I switched to AMD myself only recently, I never had a chance to witness more than one or two issues with their drivers)
The 680 has 1-2 niggling driver issues at the moment but unfortunately the 7970 is having more of a problem. If you get the 7970 just remember to never ever update to the latest drivers until they are well and truly proven to be perfectly stable. Personal experience with them has taught me to find a stable driver and stay there. (or more accurately never get a new ati card unless the series has been out over 6 months.)
The 680 has a pretty well known stable OC and does perform better than the 7970 ( which is still over priced )
Only problem with 680 id see is being wary of PhyX on the single card on high end settings (arkham - maybe a ~£40 9800gt for dedicated phyx?!) and Nvidias apparant complete disregard with LUX effects.
Imo if you can get a 680 do it, or wait a little while until the 7970 has a nice price drop.
Just keep in mind that you may get a slightly nicer OC with the non ref 7970 but that is only going to destabilize it more when you (probably) run into driver issues.