I had the 560ti, and I upgraded to the 660ti so I could play BF3 on Ultra. The 560ti is a fantastic card. Of course the 660ti is better, but is it worth $70-100 more? Some people love PhysX so they choose Nvidia over AMD.
I bought my MSI 560ti for $200, and sold it for $175. Then, I bought the 660ti for $315. Prices have dropped on the 660ti, so now its much more reasonable. If you game on 900p or less monitor then I'd say 560ti or 7850.