From the benchmarks I just looked at, they were about even, one winning in some situations and the other in others. I would choose the 7970 for the following reasons.
-less power connectors needed (most psu's don't have 4 6 pin connectors) (I would have at least 600 watts for the 7970, 650 for the 660s.)
-less microstuddering issues
-some games don't support sli or crossfire, so you would only get the performance of a 660
-1 gb extra vram
-better upgrading options
-sli and crossfire generate a lot of heat