I personally have both a 670 FTW and a reference 680, and I ran some benches on both (including BF3).
What I have concluded is: The 670 FTW at it's factory overclocked settings actually performs 4% better than the reference 680, while also staying cooler and quieter - plus its also draws less power.
I personally use the 670 FTW for my everyday gaming and use - and I picked mine up for $319 AR. I have never had a single problem with it and it's one of my favorite video cards of all time.
My recommendation (obviously) goes to the GTX 670 FTW.
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However, another approach would simply be to stay with your 7870 and OC it until either AMD's 8xxx series comes out, which is due for Q1 of 2014, or Nvidia's 7xx series, which is due for more like Q2-3 of 2014, based on this generation's release dates.