I'm not sure why you're comparing the 670 to those AMD cards, as they're in totally different price brackets.
What are your system specs? What resolution do you play at? What games do you play? Can't really answer your question without that info.
The 670's main competition is the 7970, and the 670 is worth it if you play games that prefer the 670 to the 7970 or if you just like Nvidia.