A dual 560 Ti setup is faster than a GTX 580, and completely smokes the 570. From these two options, I would go for the 2x 560 Ti. I went with dual 470s over a year ago (470 is a tad slower than 560 Ti) and I'm still rocking everything maxed out at 1920x1080 with ungodly amounts of AA and AF.
However, why not get a 580? It's not as fast as dual 560 Ti but you get the advantages of a single card (upgrade path, SLI issues go away (not that they're particularly bad), bragging rights)