SLI 570's will obviously give you more performance, so that is the winner hands down. But on the flip side the 570's IF overclocked will fry the VRM's. It's a known issue with them, but some say the non reference ones are fine to OC and have not had any issues. But if you look at the SOC Gigabyte 560Ti it performs quite well and holds it's own against a reference 580.
Or you could just break down and buy 2 GTX580's like I did yesterday
The egg has the EVGA's at $40 off plus $30 mail in rebate! It was too hard of a deal to pass up.