The 7870 by far, as the 7870 competes at a higher level than the 560ti. The 560ti competes more along the lines of the 7850, in which the 7850 is still better.
Why ask, have you found a 7870 at a similar price to a 560ti?
I am asing because i want to get a new pc and my friend asked what video card i was thinking about getting. I told him i was looking at the 7870 and he told me the 560 ti was better so i told him he was wrong and i posted this to show him that he is wrong because he is stubborn.