Now, you know that the 680 is better than 670, but the AMP model of Zotac is heavily overclocked and some sites reports in many benchmarks that has also better performances than a standard 680.
You can see in these links here
What's your suggestion? Keep in mind that the 680 is an USED vga. Owner, has bought it on April 2012, but I'm pretty sure that is in optimal conditions.