I think you should go with the GTX 660.
It's smaller and only needs one 6-pin connecter!
Performance wise there is a difference in some games, but both Card can run almost anything you throw at them at highest settings atm.
If you bought the cheaper Card, then you could add another 4 GB of ram also. http://www.youtube.com/watch?v=HaXVrCFvL94
Here is a video with benchmarking at the end, see for yourself!