radeon hd 7970 ghz is on par with a gtx 680 so it is better then a gtx 670
Without any knowledge of the rest of the rig or what usage you are going for, I would say the 7970 Ghz.
Reasons for the 670. -CUDA if any application you may use support it. -Superior 3D performance and utility -PhysX if your games support it.
Reasons for the 7970 Ghz - Extra VRAM and wider memory bus mean superior high resolution performance (and utility). Also means that you can crank up AA and MSAA. - Superior OpenCL and GPGPU performance. - Overall better performance at 1080p, will vary game to game though.