i would get the 4870X2 out of those two.
Reason is because SLI is not perfected yet, so i think a single card with 2 GPU is better than SLI/CF.
though it would be cool to say i have 2 cards in SLI....
but it will also be cool to say.. "I Have the Best Card Out Right Now Bia**h" when comparing to others. haha