I suspect that you will win more benchmarks with a GTX660 sli solution than with a single GTX670 FTW.
But, since some games do not play well with dual cards, I think your gaming experience will be better with one good card.
A GTX670FTW will also be less expensive and you will not need a sli capable motherboard or a stronger psu.
If you are running on a single 1080P monitor, you will be very satisfied with a single card.
If you want to spend all of your sli budget, you can do slightly better with a GTX680 and even get back some change.
Don't know if the small difference in performance with a GTX680 is worth it though.