Both setup's are probably pretty close to equal in terms of bang per buck. The 570's would cost more of course but would also be considerably faster.
For 1080p resolution you REALLY don't need any more than 570's in SLI ... they will max out anything, including full AA (except maybe Witcher 2 w/teh Ubersampling). A single 580 would get close to doing so, but on the toughest of games you may have to sacrifice a setting or two. For example, on Metro I don't think you could completely max it w/ADoF and 4xMSAA and everything else w/o it getting choppy at times.
So it depends on what you want to spend, really. And whether or not you want to deal with the occasional SLI hassle like when games come out and you have to wait for an SLI profile to be released ... this does happen from time to time, esp. with advance-released game demos.