I in a process of buying myself a new rig and I'm in quite a big dilemma between above mentioned cards. Now my primary use is 3D CAD modeling and rendering (SolidWorks, Cinema 4D), photo editing (PS, Lightroom), Illustrator and InDesign, but of course I'll also be using it for gaming from time to time. It seems that Nvidia optimized the Kepler family only for gaming as they lose big time on GPGPU benchmarks. Now I'm wondering how big of a difference this benchmarks would make based on my previously mentioned use. I can get a HD7950 for €330, GTX670 for €380 and HD7970 for €410 (non of them a reference design).
Based on those prices and the fact that you're going to be making use of the compute power of the card (rather than just gaming), I'd go for the 7950. With a non-reference design, it has a lot of overclocking headroom, too, which will allow it to exceed stock 7970 performance in games.