Which is more convenient to buy hd 7970 or gtx 670?

I want to buy a pc gaming like a boss but someone make me a offer and the pc have 670 but im not sure if this is the best option because i want a 7970 but whichs is better for me? 

this is the part of my computer

Intel Core i7-3770K / Quad core / 3,5GHz, Processor
Corsair Hydro H100i (waterkoeling), Processor koeler
ASRock Z77 Pro4-M, Moederboord
Corsair Vengeance 8GB DDR3-1600, Werkgeheugen
EVGA GTX 670 / 2GB / 256-Bit, Video kaart
Samsung 840 series pro 128GB ,Ssd
Western Digital Blue / 1TB / 7.200Rmp, Harde Schijf
LG CD/DVD brander, Optische drive
Windows 7 64-Bit, trial versie
Fractal Design Define R4 ,Behuizing

thanks for reading regards

Get a 7970; it's about 15% faster than a 670, at least.