The GTX 660 is a little bit faster than the GTX 650 ti boost, but I would get the ti boost if it is cheaper than the 660. (100% for the ti boost, 104% for the 660) and the ti boost can be up to $20 cheaper if you get a good deal on it.
I don't know if 650ti boost is discontinued or sold out everywhere or what...but the places I've looked like amazon the prices when available are more than the 660 costs, and closer to what the newer 760 costs lol.
660 is a little better. Just get whichever is cheaper though.