Actually it depends more on the pairs of GPU's we are using on the Crossfire or SLI.
Crossfire is the direct answer to the Nvidia's SLI.
But as far as technology is concerned,then definitely Crossfire is a bit superior than SLI.
Crossfire provides good overclocking potential to cards in comparison to SLI's! But this may also depend on the cards ised as well.
Even Power consumption is much better maintained in Crossifre setup.
As for ease of use and flexibility, nVidia has ATI beaten. nVidia’s SLI systems are much easier to set up, from both a hardware and software perspective, and there are far more choices in terms of SLI motherboards in comparison to Crossfire motherboards.
Hence, it is really difficult to judge, which is best as both companies are soon bringing up the new standards for their technologies.
But as far as current stats are concerned, then it definitely has to be NVIDIA's SLI, a true winner!!