Comparing the stats it’d be really hard to justify spending an extra $200 just for a few extra pixel pipelines and some memory bandwidth. Granted the 580DS also support 2/3/4 Way SLI but setups like that really only show their true differences in software benchmarking systems.
I’d easily say that a single 560Ti could easily handle any of today’s games at their max settings assuming the other hardware was within the same level of course. Even the cheapest GTX 580 is $439 and unless you have money to burn or no common sense the 580 series is a waste right now.
A little bit of overclocking to the 560 Ti would help put it even closer to the numbers of the superclocked 580.