A 560 is a sort of 'overclocked' 460. There's better performance, but not too much. 560's also use a little bit more power then the 460's.
If the 460's & 560's are similarly priced however, I'd probably say to just go with the 560's as they are newer and will be supported a bit longer. If there's a big price difference though, then I'd say just stick with the 460's.
The "big" differences are:
Shaders = 1024 vs 896
Compute Units = 16 vs 14
TMU's = 64 vs 56
Other then that, all other specs are the same