The 560 Ti is a faster card, definitely. But if you're thinking in terms of value, the 6870 now can be had for less than $150 with rebates. Newegg has one for $130 post-rebate: http://www.newegg.com/Product/Product.aspx?Item=N82E16814150561
That's wayyyy less than 560 Ti, which at the best prices I can find right now is still about $200. It's impossible to justify spending 50% more on the 560 Ti.
Edit: to get a sense of how comparable the cards are, check here:http://www.anandtech.com/bench/Product/290?vs=330
As you can see, there's no way the 560 Ti is 50% faster, and in some games the 6870 actually wins.
Of course you always get diminishing returns, i.e. 50% more price will never yield 50% more performance. But when those two cards are so close in performance, it doesn't really seem justifiable to me to spend so much more for such a small gain. If you really want to spend more, I'd go up to a 7850 or 560 Ti 448 core (really a totally different card than the 560 Ti). Hell, at those cards' prices, you could even crossfire 6870s if you're willing to deal with the associated annoyances.