It's probably not quite worth $100 more. It's pretty much a 460 with better power and heat management and therefore comes with higher clock rates making it a bit more powerful.
The 560 ti can be overclocked and perform similarly to that of a stock gtx 570 but that is as far as it goes. The gtx 460 is more for economy and is a mid range card. If budget and need isn't that great then go for the gtx 460 but it is not built as well as the 560ti.