Comparing BFBC2 with those two configurations, it looks like the 590 gives you an edge...http://www.guru3d.com/article/geforce-gtx-590-review/18http://www.guru3d.com/article/geforce-gtx-570-sli-review/6
But Crysis Warhead shows the 2 570s with slightly better performance:http://www.guru3d.com/article/geforce-gtx-590-review/14http://www.guru3d.com/article/geforce-gtx-570-sli-review/11
Either setup would be great. Ultimately you'd generate less heat overall with the 590 and use less power than with two 570s. The 590 would give you less noise as well.