Some things to consider...
Staryoshi did a review of the two cards when the 570 was released.
According to his results, the 570 is only ~6% faster than the 470, clock-for-clock, across all his benches. Surprising considering that's actually less
than the difference between the 560 Ti and the 560.
With that in mind, the 470 is easily the better price/performance option (as are all of the 400 series cards atm).
That said, the 470s are very close to being EOL. If you're planning on just buying one
card now and adding another later, you'd almost certainly have to buy your 2nd 470 used. They're already OOS/discontinued at most e-tailers as it is.
The 570 is faster, cooler, more efficient, clocks higher on average, and has a longer shelf life. And if you're just buying one now, you'll probably be able to score your second card at a much lower price if you wait it out (Black Friday sales anyone
I think I'd lean towards the 570 personally.