Are you sure you don't mean the Ultra-D? I could've sworn DFI only had one SLI board, the SLI-DR, and the Ultra-D was single PCI-E. Although the Ultra-D you can mod pretty easily into a 16x/2x SLI board if you wanted to.
They are both about the same, if you wanted to save some money I'd buy the Ultra and just mod it, but as far as performance goes the 8x/8x has its up and downs and the 16x/2x has its ups and downs.