The 560 will come with an 820 core clock and 384sp. Expect it to perform about 10% faster than a 460 at 820 core will.
I would say the 460 is the much better choice, the 560 is expected to be set to come in at the $250-$275 price point.
Expect to see the same situation you see comparing a 580 to a 480, 15% faster. The caveat to that being at the same clocks a 580 is only 5% faster than a 480, and the bulk of it's performance over a 480 is a higher clock speed, that will not be the case with the 560 as it has a bigger shader bump over a 460 than a 580 does over a 480. But it will not get the kind of overclocks 460s can which usually get an easy 200-250 more. Don't expect to see 560s all running at 1000+ with the same ease.
Just buy a 460 and overclock it for much less cash. If you can get it 900 core, it will be faster than a stock 560, if you can do 1000 core (unlikely) it will be faster than even many overclocked 560s.
Pricewise 460 - $150, 560 - $250 at the least. $100 more for 20% more performance, not worth it.