My 7970 can go above 1200MHz, I think most can get there about, many even to around 1300MHz. The general concensus appears to be that the GTX680 is also a fairly capable overclocker, going by posts I've read anyway. Just a guess on my part, but I would imagine a your average max overclocked 7970 might have a very slight lead compared to the average overclocked GTX680, but it would likely be the type of lead you only see in benches. Game play experience would likely be very, very close with either.
So with that, I would focus more on the other things since performance is likely to be very much a wash. Do you want the compute performance that the 7970 offers? The extra 1GB or ram? Or does a 4GB GTX680 sound even better? The GTX680 is more power efficient, maybe that's a consideration? Physx/CUDA? Reference 7970's are too loud when overclocked and overvolted, in my opinion (speaking from experience). Look at non-reference cards, see what has the warranty you like, any extras, etc.
I don't think you'll go wrong with either card when looking at just the performance aspect. Because they're likely to be so similar with performance I would look at those other things and put more stock into those when making a decision on one. Just my $.02.