sizzling :
Is your 670 a 2gb version? To me there is little point in sli if it mean being restricted to 2gb VRAM these days. Games even at 1080p often use more than 2gb if you max them out and a handful use more than 3gb (found this with my 780) so I expect 2gb could be quite limiting in the near future
Without any doubt the SLI 670's, when directX12 will be released you'll have 2x2GB of Vram instead of just 2GB.(which i thought is not yet confirmed though..)
Otherwise i'd still go with SLI 670's i think.