SLI now has True x16, which makes both cards run at Full PCI-Exprees x16 speeds, as oppossed to one card being x16 and the other at x8. However!
ATI has way more options when you link both GPUs together. Such a mode would be SuperTiling, in which each card processes a certain area of the screen, or Super Anti-aliasing, Alternate Frame Rendering, Scissor, and another mode I believe. When it comes to dual-GPUs, ATI is king.