The 7870 is much more efficient and offers newer features than the 570. The power consumption should be half of that of the 570 as well as being faster. It has more overclock headroom and should be cooler. Unless the 570 is more than $70 less than the 7870 I would say the 7870 is the better deal, this is if they both are reference clocks and have decent coolers.
I have a 570 and it performs absolutely fine, I play Battlefield, Crysis 2 and Skyrim on it and I have modified ini config on skyrim for better quality and I still run it fine and I have a super bad CPU aswell lol here is the GTX 570 (factory overclocked) on battlefield 3, but to the guy that compared a OC'd GPU to a reference is a bit unfair, I think that you should calculate the percent overclocked and overclock them both and then compare, it's like me saying that my 570 Classified OC'd has better performance than a 7870. I know that it has a better overclocking capability but I don't like overclocking my graphics card, so calculating the percent difference is a better test.