Provided that you have a system which has the capability of running either setup without bottlenecks at high resolutions, you've got to ask yourself two things- how much you're willing to spend and do you really want a multi-GPU setup from the beginning?
Having said that the GTX 570(350 $) is cheaper than 2 6870s(480$).
The GTX 570 will play almost all games at highest settings and high resolutions.So, will the 2 6870s, which will be even faster than the 570.
However you'll have to consider the fact that not every game will scale properly with multi-GPU setups.
Thus, go for the 570 if you want to save money or 2 6870s if you want raw performance from the beginning.