i dont deny theres 10% i dont see 20% anywhere in performance gap between the gtx 660ti and 670, maybe in the future when games will fully utilize the bigger bus and extra 1gig of vram, but until than the gap isnt that great, it also comes down to a money thing, i can get a gtx 660ti for 230$+tax here and the 670 starts at 356$, thats 120$+tax+shipping difference, i really cant justify spending that much since im on a budget.
but yeah u get more bang for bucks with the gtx 660ti and if u got extra buy a gtx 670 or a 7970 on sale, seen a few at 360$ and those beat both the 670 and 680 series.