The 580 is the better card for the reasons smarteyeball pointed out.
However, if 1) Price is a major concern & 2) the rest of your PC is somewhat dated
Then the difference in price may not be justified or you will not be able to benefit from what it offers.
It really comes down to what your putting it in and your expectations.
I to have a eVGA 570 SC, and its a amazing GPU. Runs everything I play at higher settings without issue, and i can honestly say 1 of the best GPUs Ive ever had.
Either of these two cards will be a good choice though IMHO.
Now if you are looking towards the highest end GPU, it may also be worth considering the GTX 680. Much more powerful than the 580, and about 60-70$ USD more.
If your going to spend $430ish on a GPU, another $60 is not really that much more in the grand scheme of things.
Anyway, for the price the 570 is amazing, you will not be disappointed. The 580 is better, but really your call if its worth the extra and if you really need it.
Chips,
This report shows you the difference of a reference 570 and a reference 580. Now, keep in mind there are a sea of versions beyond reference for both cards. I know you are looking at a full build, and you want to do the most with your money, so that being said, you should basically get the most powerful card you can possibly afford. After you have finalized all of your choices, look at your budget, and see how far you can stretch it. A single 580 is always going to be better that a single 570. As I had previously mentioned to you, brand is important. A lot of the members, myself included, recommend EVGA for Nvidia GPUs. They have amazing customer service, and I have yet to have one of their cards fail.
James