I tested the 7970's in crossfire on single 1920x1200 and 5760x1200. I have done the same thing with the 680's, but I have only had them a few weeks and I have found myself playing a lot of games triple monitor where on the 7970's it wasn't possible.
NVidia in SLI >> ATI in xfire for 1920x1200
NVidia in SLI >>> ATI in xfire for 5760x1200
NVidia > ATI in 1920x1200
the NVidia's in SLI are not as smooth as on their own. It starts to show some stutter around 45 fps, where a single 680 looks quite smooth down to 30 fps if not a little lower. Which says if you get less than 50% extra from the second card you would be better turning it off! But on AMD anything less than or greater than 60 fps would stutter worse than anything I have seen on NVidia. Honestly I would call AMD in crossfire on 7970's unplayable. Its not just microstutter, its simply does not fool me there is motion properly. It gave me headaches .
So SLI is worse than a single card, but all NVidia options are better than the AMD ones, single or xfire. NVidia is just smoother at lower FPS than AMD. No amount of overclocking my 7970s solved the problem, they stutter even only when loaded to 30%. They just sucked donkey balls in games and in Windows they have such massive performance problems you need to turn transparency off and then you'll "only" have 300ms of input lag.
Get the NVidia card(s). Its not that AMD's cards don't benchmark well, its that the benchmarks aren't measuring what actually matters.