i'd go with 680, it was totl that time, 280x was not if i am not mistaken. that is based on what i know of their performance, but if the other is cheaper then it might go the other way
The 680 IS Slightly Better In Performance in in most games but the 280x will jump ahead in a few because of its extra Gig of Ram So dont spend more for a 280x I can currently get one used for 160 and a used 280x for 180ish
Plus for the future you can get a second one cheap too sli which is slightly better in terms of compatibility versus the crossfire in amd's cards which is only slightly behind but still enough that it matters