If you're going for a two-card setup and these are the choices, I'd choose the 460x2. I've done crossfire and dealt with the driver issues. I'll need some serious convincing to ever go back to AMD when it comes to using more than one card.
SLI, so far, has worked flawlessly for me. That's why I recommend the two 460s.
Actually, too, if you're considering the cost of two 6870s, you could push it $30/card and why not go 560x2?