The GT 260 is more powerful, but won't run modern DX11 games, so the GT 640 is automatically better. It's not a very good card but for less demanding games like free to play, indies or old games it's good.
The 560Ti is a good card, but quite old now, if you can stretch to a R7 260x 2GB or 750Ti 2GB, good. Otherwise the 560Ti will play modern games too, just not as well.