Well both AMD and NVIDIA put out compelling products that appeal to different people. You have to consider a few things when looking into a video card.
1. 1 screen or more
5. Screen Resolution.
Personally what I look for in a card that I'm wanting to buy is what will be the best card to serve my needs. So that includes maybe games that I play what would give me the best overall visual and performance experience. Lately it has been with NVIDIA cards mainly because of the games I play however I have loved my AMD/ATi cards of the past. I do check ram I would like 2-4 gigabytes of ram and I also look at memory bit I used to check CUDA cores with the 500 series but with the 600 series you have cards at the high end that all share the same amount 660Ti has the same as the 670 the 680 has more cores but I try and get the best card without getting the most expensive.