Regardless of that, I will suggest the 660 Ti, because I always recommend a single card over a dual card configuration as I am not a fan of SLI or CF. So, get yourself a 660 Ti. I have heard many driver or overheating problem with dual card (CF or SLI) configs.
If you have a 560 Ti already, That will be another different story. I think you should get another 560 Ti instead.