As of now, CF scaling and performance is better than SLI.
Sli was always better than cf with the exception of this generation gpu's. Cf is scaling very well now and is par with sli and sometimes even surpasses it.
This generation, amd gpu's scale better because of the extra framebuffer their cards come with relative to their nvidia counterparts.
Let me give an example:
We all know that gtx 570 is a stronger gpu than 6950. Its ahead in almost all benchmarks. But at higher resolutions the gap closes pretty much. In their respective multi gpu setups, 6950 cf performs and scales better than 570 sli because the former comes loaded with 2gb vram while the latter only has 1.25gb.
So the 570 is held back at higher resolutions and cannot scale as good as a 6950 duo. Nvidia must take this matter seriously on its future gpu's.
Some board makers are taking the initiative and making custom designs with extra vram. The zotac and palit gtx 580's come with 3gb vram instead of 1.5gb on the reference design.
If we compare 580sli with 6970cf, the 3gb version will increase the performance gap between 580 and 6970 at higher resolutions and allow the gpu to perform more efficiently without sacrificing fps.