That 560 is a low end card, the 660 gtx is a mid-end card and from a newer generation.
The 660 gtx is much better than the 560 gtx, 560 ti and even 570gtx.
It has the triple of the cuda cores and has consumes less power.
The obvious choice will be the 660 gtx.
The only card who can do better but not by much is the 7870 and it is at a lower price, but by the looks of it you prefer nvidia.