Crossfire is arguebly better though, as it works on all DirectX 3D applications, unlike nVidias' SLI that only works on games that are in the Driver database (which is updated with every newly released driver).
I think many people overlook this limitation.
Here's the list of supported nVidia SLI applications. just 400 in all.
Some of which are just pathetically never going to need SLI because of the age of the games. This is the reason I simply can't stand SLI. To me it's a marketing ploy, as the game developers probably need to license or pay some sort of royalties to nVidia for SLI support weather it be for cash or free advertising, who knows. nVidia could easily, just like ATI, impliment SLI at DirectX API level to support all DirectX games and apps.
Another advantage is if a new game comes out, you don't need to wait for SLI to support it, Crossfire automatically does.