First off, you wanted to know which is better.
For the price, I'd go with ATi, because you can buy an HDD 5770 1GB that supports DX11 for under 200$, nVidia has a 400 series card line out that has DX11 but is a lot more spendy.
SLi - Scaleable Link Interface
Crossfire - Cards running together to even out the workload between GPU's, enhancing performance.
SLi is very card specific, your GPU's need to be the SAME in order to run SLi.
Crossfire is not specific, this is why crossfire is, in my opinion, more efficient than SLi.
Crossfire takes video cards from ATi, and no matter what their clock speeds or VRAM is, it splits the workload evenly for each card to perform better, and it is a very nice feature. You could have a 1gb 5770 and a 512mb 5770 and basically just run that to have a 'super card' in this aspect that runs at 1,512 or 1,524 MB, I forgot.
That's just my two cents, I'm not sure if this information is 100% amazing, but it's just my knowledge on the subject.