Definetly the 7970, great OC potential, and is in more of a range of a 680 often and at times even beat it.
Depends on what you want in the future. Both cards are really good price/performance wise. 7970 is more powerful, and is better for multi-monitor gaming The 670 is more power efficient and better for 3D gaming.
the hd 7970 is very slightly better than the gtx670