Well EVGA 680 is top class, but what your final decision will be depends on what use are you gonna make to your pc. 680 outbids 670 and the price difference is big. If you are going for hard gaming I believe you should go for the 680 (I am in love with it BTW
). Comparing the two (ZOTAC and EVGA) I would stick to the EVGA not just for the brand but in terms of gpu capabilities as well. Though the human will not notice at all the difference in performance between the two gpu's I would give that extra 20 bucks to get a theoretically better card.
Plus the EVGA comes with a backplate which provides structural support and more effective cooling!
Hope this helps!