I prefer ATI over NVidia since their Radeon 9000 series was released. In my opinion their cards are more powerful. NVidia's Geforce 5000 series has has it's share of problems but not the Radeon 9000 series. The latest Radeon cards like the X800 and X850 are better than the Geforce 6800. Nvidia recently released their 7800GTX which is better than any card out right now. I am sure though that the next ATI cards will beat it.
Nvidia has SLI, but it doesnt work with every game. ATI promisses that CrossFire will be compatible with any games and you wont have to buy an exact same card for it to work. We will have to wait until this is out before we can say who's best for dual GPUs. If ATI delivers what they promise, ATI will be best.