SLI Remains Superior
So why not recommend ATI's CrossFire? For starters, the whole external cable connector thing still really bugs a lot of users, it's nowhere near as elegant as SLI's implementation. And the fact that you have to pay more for the CrossFire Master Edition card than for the standard version.
Nvidia supports mixxed vendor support too, so you can search and combine 2 different brands (the same type of card though, with equal memory) for your personal requirements. This could come in handy when you don't buy 2 cards at the same time, prices on certain brands tend to change fast.
Finally, unless you're a die-hard ATI fan with lots of money, the results just don't warrant the cost (of course, you can probably make the same argument about high-end NVidia cards and SLI but Nvidia offers SLI on cheaper models too).
Both setups deliver excellent image quality, but when it comes to brute power, an Nvidia SLI setup walks away the clear winner.