logainofhades :
http://www.anandtech.com/bench/product/780?vs=777
I was just about to post the same link hehe.
Well as you can see the differences, now you might decide what are you gonna get.
I think you should still go for the AMD Radeon HD 6870, as it preforms better at the newer games.
If you are not on a budget or even if you are but can go a bit over it, you shall get an R7 260X or 265.
One of those two will be better than both of those what you listed at the title, as it's a recently released GPU and it has better performance.
Hope I helped and answered your question, so you can make a decision now
Regards.